Cyclical mastalgia is very common in Western populations and is believed to have an hormonal basis. Simple measures such as vitamins or evening primrose oil are not very effective, yet the disease rarely warrants anti-oestrogen therapies. Isoflavones are a subgroup of phytoestrogens which we hypothesized might be a simple and effective means of therapy as they act as a weak anti-oestrogen in pre-menopausal women and have no side-effects. A double-blind randomized control trial of either placebo, 40 mg or 80 mg of isoflavones was undertaken after an initial 2 month single-blind 'Placebo Lead-in' to exclude women with a significant placebo response. Eighteen women were randomized to the treatment phase of the trial. Nine of the 12 women on treatment had a worthwhile improvement in their pain compared to only two of six on placebo. The reduction in pain was 13% for placebo, 44% for 40 mg of isoflavone per day and 31% for 80 mg per day. There have been no previous clinical studies of isoflavones for the treatment of mastalgia and the benefit demonstrated in this study adds another valuable arm to therapy.