diff --git a/Server.go b/Server.go
index 093080c..477b8e8 100644
--- a/Server.go
+++ b/Server.go
@@ -8,6 +8,7 @@ import (
 	"os"
 	"regexp"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/boltdb/bolt"
@@ -44,6 +45,7 @@ type Server struct {
 	opts           ServerOptions
 	db             *bolt.DB
 	startTime      time.Time
+	thumbnailSem   sync.Mutex
 	metadataBucket []byte
 }
 
diff --git a/thumb.go b/thumb.go
index cfcc3bb..556a208 100644
--- a/thumb.go
+++ b/thumb.go
@@ -1,6 +1,7 @@
 package contented
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"log"
@@ -51,6 +52,8 @@ func getThumbnailerConfig(t byte) (*thumbnail.Config, error) {
 }
 
 func (this *Server) handleThumb(w http.ResponseWriter, r *http.Request, thumbnailType byte, fileId string) {
+	ctx := r.Context()
+
 	opts, err := getThumbnailerConfig(thumbnailType)
 	if err != nil {
 		log.Printf("%s Thumbnail failed: %s\n", this.remoteIP(r), err.Error())
@@ -58,9 +61,18 @@ func (this *Server) handleThumb(w http.ResponseWriter, r *http.Request, thumbnai
 		return
 	}
 
+	// Only calculate one thumbnail at a time
+	this.thumbnailSem.Lock()
+	defer this.thumbnailSem.Unlock()
+
+	if ctx.Err() != nil {
+		// The request was already cancelled
+		return
+	}
+
 	t := thumbnail.NewThumbnailerEx(opts)
 
-	err = this.handleThumbInternal(w, t, fileId)
+	err = this.handleThumbInternal(ctx, w, t, fileId)
 	if err != nil {
 		log.Printf("%s Thumbnail failed: %s\n", this.remoteIP(r), err.Error())
 
@@ -69,7 +81,7 @@ func (this *Server) handleThumb(w http.ResponseWriter, r *http.Request, thumbnai
 	}
 }
 
-func (this *Server) handleThumbInternal(w http.ResponseWriter, t thumbnail.Thumbnailer, fileId string) error {
+func (this *Server) handleThumbInternal(ctx context.Context, w http.ResponseWriter, t thumbnail.Thumbnailer, fileId string) error {
 
 	// Load metadata
 	m, err := this.Metadata(fileId)