SUMMARY—Biochemical studies were performed on 80 adult males with renal stones and 24 controls from Northern Sudan. Similar studies were performed on 44 controls from Southern Sudan, where urinary stones rarely occur.No significant differences between stone formers and controls from Northern Sudan were found regarding serum uric acid, phosphate, alkaline phosphatase, total protein, albumen, daily urine calcium or uric acid. Similarly, no significant difference was found between the 2 groups in the incidence of hypercalciuria, hyperuricaemia or hyperuricosuria. The serum calcium was significantly higher in stone formers.The mean serum and daily urine calcium were significantly lower, and the mean serum and daily urine uric acid were significantly higher in controls from Southern Sudan compared to either stone formers or controls from Northern Sudan. The mean daily urine volume was significantly higher in Southern controls compared to Northern controls.The increased prevalence of stone formers in Northern Sudan appears to result from the higher calcium output in the urine and the lower urine volume in the North compared with the Sout