To understand the reasons for underreporting of occupational cancers, we investigated cases' perceptions of the causes of cancer. As part of a case-control study in the province of British Columbia, Canada, 105 bladder cancer cases, 48 nasal cancer cases, 51 mesothelioma cases, and 159 population-based controls (frequency matched to cases on age and sex) were interviewed using structured questions about their smoking, medical, residential, occupational, and carcinogen exposure histories. We asked cases what they thought caused their disease, and asked population controls about their understanding of the etiologies of the three cancers. Most cases and controls (69%) indicated that they had ''no idea about causes, with the exception that the majority of mesothelioma cases (70%) recognized asbestos as a cause. Smoking was perceived as a cause of bladder cancer by 21 percent of cases. Many cases knew about the asbestos and smoking etiologies from discussions with their physicians. Chemicals were commonly cited as causes of nasal and bladder cancer, but very few specific known or probable carcinogens for these sites were named. Cases (12%) more frequently than controls (2%) thought prior disease or trauma was a cause for all three types of cancer. Other etiologic factors less frequently listed by subjects included environmental pollution, hereditary factors, drugs, and radiation. Most cases recognized the major cause of mesothelioma, but few subjects knew about lifestyle or occupational causes of bladder cancer or nasal cancer, suggesting that education about the multiple cancer risks of smoking and about occupational carcinogens needs to be improved.