We investigated the prevalence of stone disease and urinary stone risk factors in machinists chronically exposed to a hot environment and massive sweating, without interference of nephrotoxic metals or other lithogenic compounds. The study was performed at a glass plant and exposure to heat stress was estimated by the Wet Bulb Globe Temperature climatic index. The prevalence of nephrolithiasis on the entire population of the machinists was 8.5% (20 of 236), while the prevalence on the controls working in normal temperature was 2.4% (4 of 165) (p = 0.03). A high incidence (38.8%) of uric acid stones was present in the workers exposed to heat stress. Among the urinary stone risk indexes determined for 3 days during the 8-hour work shift on a randomly selected sample of 21 workers exposed and 21 workers not exposed to heat stress without any evidence of stone disease significant differences were found in uric acid concentration (722 +/- 195 versus 482 +/- 184 mg./l., p < 0.001), specific gravity (1,026 +/- 4 versus 1,021 +/- 6, p < 0.005) and pH (5.31 +/- 0.28 versus 5.64 +/- 0.54, p < 0.02), respectively. Thus, high uric acid relative supersaturation was present during occupation in hot temperatures (8.67 +/- 3.49) compared to occupation in normal temperatures (4.15 +/- 2.7) (p < 0.001). This study confirms that chronic dehydration represents a real lithogenic risk factor, mainly for uric acid stones, and adequate fluid intake is recommended during hot occupations.