One hundred sixty incident cases of renal cell carcinoma under the age of 55 years and an equal number of age-(within 5 yr), race-, and sex-matched neighborhood controls were interviewed. Cigarette smoking was a risk factor for renal cell carcinoma in males [relative risk (RR) = 2.1, one-sided P = .02] but not in females (RR = 1.1, P = .50). Obesity was a significant risk factor in both males and females: The RR for a 4th quartile compared to a 1st quartile Quetelet's Index 10 years ago was 2.5 for males and 3.3 for females. Additional risk factors in females were diuretic use (RR = 4.5, P = .002) and daily coffee consumption (RR = 2.3, P = .06).