Concentrations of two small stress proteins, αB crystallin and the 27-kDa heat shock protein (HSP27) were quantitated in tissues of the human normal genitourinary system and their tumors. Levels of HSP27 in renal cell carcinomas (mean ± SE: 1450 ± 262 ng/mg protein, n = 15) were significantly higher than in normal kidney (the cortex: 540 ± 99 ng/mg protein, n = 13; the medulla: 600 ± 106 ng/mg protein, n = 13) while those of αB crystallin tended to be increased without statistical significance. These findings were similar to those previously reported for renal cell tumors chemically induced in rats. Concentrations of αB crystallin in prostatic carcinoma tissues (410 ± 129 ng/mg protein, n = 10) were also significantly higher than in benign prostatic hyperplasia (54 ± 12 ng/mg protein, n = 14), whereas αB crystallin levels in testicular tumors including seminomas (2.1 ± 0.8 ng/mg protein, n = 11) and non-seminomas (5.2 ± 2.3 ng/mg protein, n = 9) were significantly lower than in normal testicular tissues (29.7 ± 6.2 ng/mg protein, n = 5). Both αB crystallin and HSP27 could be immunohistochemically localized in the normal kidney and renal cell carcinoma tissues.