diff --git a/cmd/syncthing/blockprof.go b/cmd/syncthing/blockprof.go
new file mode 100644
index 000000000..86af6ae48
--- /dev/null
+++ b/cmd/syncthing/blockprof.go
@@ -0,0 +1,59 @@
+// Copyright (C) 2014 The Syncthing Authors.
+//
+// This program is free software: you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// This program is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+// more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program. If not, see <http://www.gnu.org/licenses/>.
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/pprof"
+	"syscall"
+	"time"
+)
+
+func init() {
+	if innerProcess && os.Getenv("STBLOCKPROFILE") != "" {
+		profiler := pprof.Lookup("block")
+		if profiler == nil {
+			panic("Couldn't find block profiler")
+		}
+		l.Debugln("Starting block profiling")
+		go saveBlockingProfiles(profiler)
+	}
+}
+
+func saveBlockingProfiles(profiler *pprof.Profile) {
+	runtime.SetBlockProfileRate(1)
+
+	t0 := time.Now()
+	for t := range time.NewTicker(20 * time.Second).C {
+		startms := int(t.Sub(t0).Seconds() * 1000)
+
+		fd, err := os.Create(fmt.Sprintf("block-%05d-%07d.pprof", syscall.Getpid(), startms))
+		if err != nil {
+			panic(err)
+		}
+		err = profiler.WriteTo(fd, 0)
+		if err != nil {
+			panic(err)
+		}
+		err = fd.Close()
+		if err != nil {
+			panic(err)
+		}
+
+	}
+}
diff --git a/cmd/syncthing/main.go b/cmd/syncthing/main.go
index 2c6fa6ef5..d2e8de382 100644
--- a/cmd/syncthing/main.go
+++ b/cmd/syncthing/main.go
@@ -142,38 +142,41 @@ Development Settings
 The following environment variables modify syncthing's behavior in ways that
 are mostly useful for developers. Use with care.
 
- STGUIASSETS   Directory to load GUI assets from. Overrides compiled in assets.
+ STGUIASSETS     Directory to load GUI assets from. Overrides compiled in assets.
 
- STTRACE       A comma separated string of facilities to trace. The valid
-               facility strings are:
+ STTRACE         A comma separated string of facilities to trace. The valid
+                 facility strings are:
 
-               - "beacon"   (the beacon package)
-               - "discover" (the discover package)
-               - "events"   (the events package)
-               - "files"    (the files package)
-               - "net"      (the main package; connections & network messages)
-               - "model"    (the model package)
-               - "scanner"  (the scanner package)
-               - "stats"    (the stats package)
-               - "upnp"     (the upnp package)
-               - "xdr"      (the xdr package)
-               - "all"      (all of the above)
+                 - "beacon"   (the beacon package)
+                 - "discover" (the discover package)
+                 - "events"   (the events package)
+                 - "files"    (the files package)
+                 - "net"      (the main package; connections & network messages)
+                 - "model"    (the model package)
+                 - "scanner"  (the scanner package)
+                 - "stats"    (the stats package)
+                 - "upnp"     (the upnp package)
+                 - "xdr"      (the xdr package)
+                 - "all"      (all of the above)
 
- STPROFILER    Set to a listen address such as "127.0.0.1:9090" to start the
-               profiler with HTTP access.
+ STPROFILER      Set to a listen address such as "127.0.0.1:9090" to start the
+                 profiler with HTTP access.
 
- STCPUPROFILE  Write a CPU profile to cpu-$pid.pprof on exit.
+ STCPUPROFILE    Write a CPU profile to cpu-$pid.pprof on exit.
 
- STHEAPPROFILE Write heap profiles to heap-$pid-$timestamp.pprof each time
-               heap usage increases.
+ STHEAPPROFILE   Write heap profiles to heap-$pid-$timestamp.pprof each time
+                 heap usage increases.
 
- STPERFSTATS   Write running performance statistics to perf-$pid.csv. Not
-               supported on Windows.
+ STBLOCKPROFILE  Write block profiles to block-$pid-$timestamp.pprof every 20
+                 seconds.
 
- STNOUPGRADE   Disable automatic upgrades.
+ STPERFSTATS     Write running performance statistics to perf-$pid.csv. Not
+                 supported on Windows.
 
- GOMAXPROCS    Set the maximum number of CPU cores to use. Defaults to all
-               available CPU cores.`
+ STNOUPGRADE     Disable automatic upgrades.
+
+ GOMAXPROCS      Set the maximum number of CPU cores to use. Defaults to all
+                 available CPU cores.`
 )
 
 // Command line and environment options