The beneficial effect of allopurinol treatment in metaphylaxis of hyperuricosuric calcium oxalate lithiasis has been analyzed in a retrospective study covering 10 years. 83 patients with hyperuricosuric calcium oxalate lithiasis (48 recurrent stone formers, 35 patients with a single stone episode) were treated with 300 mg allopurinol for 33.1 +/- 11.5 months. Subsequently, each patient was controlled for 43.9 +/- 27 months without therapy. The overall relapse rate was 28.9% but there is no statistically significant difference between treatment with and without allopurinol (Mann-Whitney: p = 0.16). The only striking difference was the fact that, with one exception, stone relapses occurred only in the group of recurrent stone formers, whereas the group with single stone formation seemed to be at a clearly decreased risk for stone recurrence during follow-up. This would suggest that factors other than those investigated in this study are responsible for the phenomenon of recurrent stone formation.