The treatment of established cystoid macular edema has been enigmatic. This is a preliminary study of five patients treated by intermittent hyperbaric oxygen with an intensive regimen of 1.5 hours two times per day for seven days and two hours per day for an additional 14 days. Visual acuity improved within 14 days in all five patients: One patient improved from 20/40 to 20/15, one from 20/50 to 20/25, one from 20/200 to 20/40, one from 20/70 to 20/25, and one diabetic patient improved from 20/70 to 20/25. Vision has tended to regress with time.