Objective To identify biochemical and dietary factors (P<0.001) in black than in white subjects. Urinary potassium, calcium, citrate, phosphate and cystine which may play a role in the low incidence of stone formation in the black South African population.were all significantly lower in black than in white subjects (P<0.001 for the first four and P<0.03 Subjects and methods The study included 31 semiurbanized black and 29 urbanized white subjects. The for cystine). Conclusion Certain intrinsic factors in South African protocol and modern laboratory techniques used to assess recurrent stone formers were followed. Urinary black subjects may account for their lower frequency of stone formation than in white subjects. Of these, sodium, potassium, creatinine, calcium, phosphate and urate levels were measured, and urinary citrate, the very low urinary calcium, decreased urinary cystine and diCerent interactions between sodium and oxalate and cystine assessed. Results Black subjects ate a diet significantly higher in calcium/cystine are probably important. Keywords Epidemiology, ethnic diCerences, black, white, sodium (P<0.04); there was no diCerence in serum levels but urinary sodium was significantly higher control, urolithiasis

Although the urine of BSF tends to be more like that of WSF, the lithogenic risk factors present differed and were less severe. The increase in urinary calcium excretion, coupled with a lower citrate excretion, may be the most important reason for stone formation in the black population. An increased protein intake may also play a role. In this small group of BSF there was no family history of urolithiasis, supporting the view that there may be a more prominent genetic influence in WSF.