Hyperuricosuric patients may form uric acid kidney stones and mixed stones containing both calcium oxalate and uric acid. Some of these patients form pure calcium oxalate stones. Explanation of this syndrome was based on the plausibility of epitaxial growth of calcium oxalate on uric acid crystals acting as substrates. In spite of convincing crystallographic consideration, laboratory experiments did not demonstrate any growth of calcium oxalate on uric acid seeds. An amino acid evidently adsorbing on uric acid seeds and attracting calcium ions could act as a mediating agent, thus realizing the potential of the epitaxial growth of calcium oxalate on uric acid crystals. Administration of allopurinol to hyperuricosuric calcium oxalate stone formers reduced the level of uric acid, consequently preventing the creation of uric acid crystals in urine. It should have removed the direct cause for the formation of calcium oxalate stones. Though undoubtedly more effective than placebo, the therapy with allopurinol was not unequivocally successful. Combined therapy using allopurinol and other drugs which were proved to be beneficial for idiopathic calcium oxalate stone formers, seems to give improved results. The use of procedures for evaluating the effect of therapy on risk factors has been started to predict success in individual cases.