diff --git a/pwd/pwd.go b/pwd/pwd.go
index 99d5fde..6714b02 100644
--- a/pwd/pwd.go
+++ b/pwd/pwd.go
@@ -1,6 +1,7 @@
 package pwd
 
 import (
+	"sync"
 	"time"
 
 	"github.com/play-with-docker/play-with-docker/docker"
@@ -23,6 +24,7 @@ var (
 )
 
 var sessions map[string]*Session
+var sessionsMutex sync.Mutex
 
 func init() {
 	prometheus.MustRegister(sessionsGauge)
diff --git a/pwd/session.go b/pwd/session.go
index 6114680..89d0ca2 100644
--- a/pwd/session.go
+++ b/pwd/session.go
@@ -52,6 +52,9 @@ type Session struct {
 }
 
 func (p *pwd) SessionNew(duration time.Duration, stack, stackName string) (*Session, error) {
+	sessionsMutex.Lock()
+	defer sessionsMutex.Unlock()
+
 	s := &Session{}
 	s.Id = uuid.NewV4().String()
 	s.Instances = map[string]*Instance{}