Two hundred women with breast cysts proven by aspiration were entered into a randomized double-blind trial of Efamol (evening primrose oil) at a dose of 6 capsules daily or equivalent placebo dose for a year. Cysts were categorized by initial electrolyte composition, and follow-up continued for 1 year posttherapy. Recurrent cyst formation in the first year was slightly (but not significantly) lower in the Efamol group compared with the placebo-treated group. The Efamol treatment was well tolerated as the dropout rate was only 7% and equal in both the active and placebo groups. The initial electrolyte composition did not predict for cyst recurrence.