OBJECTIVE To assess the efficacy of alkali citrate therapy in patients suffering from recurrent idiopathic calcium oxalate stone formation. METHODS Fifty patients suffering from active stone formation who had had at least one stone annually over the previous 3 years were recruited and randomly divided into two groups. They were given either general prophylactic instructions, i.e. abundant liquid intake and dietary restrictions (group I, n = 25)--or were additionally treated with alkali citrate (group II, n = 25). RESULTS In group I, the rate of stone formation was reduced from 1.8 to 0.7 stones per patient per year. Similar results were obtained for group II, the corresponding figures being 2.1 and 0.9. There was a statistically significant higher urinary citrate excretion in group II than in group I. No difference was seen between the two groups regarding recurrent stone formation (Student's t-test). Stone formation decreased in both groups (group I 27%, group II 31%). Subjectively, 56% of group II patients, all of whom had previously experienced severe colic, reported spontaneous stone elimination to be painless, whereas in group I all but one patient suffered pain. CONCLUSIONS An objective benefit of alkali citrate could not be established.