Spaceflight could provoke formation of kidney stones, in part by causing hypercalciuria and hyperphosphaturia. Applicants for spaceflight who have metabolic or environmental derangements to begin with might be particularly susceptible to stone formation in space. We, therefore, analyzed 24-h urine samples for stone-forming risk factors in 104 male applicants before their selection into the astronaut-mission specialist corps. The urinary environment was abnormally supersaturated with calcium oxalate in 25.0% of applicants, brushite in 36.5%, and monosodium urate in 66.3%, predisposing these applicants to crystallization of stone-forming calcium salts. This high level of supersaturation was caused by both "metabolic" and environmental disturbances. Thus, hypercalciuria was found in 11.5% of applicants, hyperoxaluria in 2.9%, hyperuricosuria in 18.3% and hypocitraturia in 5.8%. Environmental derangements were generally more prominent, as indicated by low urine volume of less than 2 L.d-1 in 84.6%, high urinary phosphate in 24.4%, and high urinary sodium in 10.6% of applicants. The results suggest that most of the abnormal stone risk factors disclosed among applicants for spaceflight programs were environmental in origin.