Simple wrist ganglions were studied by carpal arthrography in fifty-nine patients. In about two thirds of these the contrast medium filled the ganglions, and tortuous narrow duct connections with the carpal joint cavity were seen. By injecting the ganglion, however, no such connection could be demonstrated. In several patients the joint capsule had additional deep and narrow pouches, probably normal anatomical variations, which were considered to be preganglionic changes. It is concluded that a valve mechanism exists which prevents the return of the joint fluid, a condition necessary for ganglion formation.