Sixty-four cases of renal cancer in white males were compared with controls (197 cases of nonmalignant diseases of the digestive system and 72 cases of colon cancer) for past exposures to cadmium. Controls were also white males, and were group-matched to the cases on age for the analyses. Data on the three main sources of exposure to cadmium--diet, cigarette smoking and occupation--were obtained by interview. The results showed a significant association of renal cancer with exposure to cadmium, and favored a synergistic effect between occupational exposure and smoking. The relative risk for men who both smoked and worked in high-risk occupations was more than four times that for men who did neither. The apparent synergism, however, suggests that the agent in cigarette smoke contributing to this association may not be cadmium.