SummaryTwenty-six patients with Baker's cysts have been observed in the period January 1963 to May 1964. Swelling, redness, tenderness and pain in the leg below the cyst was noticed in 11 patients. Seven of these had anti-coagulation treatment before admission. No certain symptoms of thrombophlebitis were found, however, and the anti-coagulation therapy was stopped. Treatment was individualised and the effect of combined injections of thio-tepa and hycrocortisone suspensions into the cysts and intra-articularly seemed to give better results than injections with hydrocortisone alone. Operative therapy is also mentioned.