The risk of cancer was evaluated among 77,952 asthma patients with bronchial asthma. The series was obtained through linkage of two registers: the Finnish Social Insurance Institution's file of asthma patients and the Finnish Cancer Registry. There was a significant excess risk of lung cancer in both sexes, the standardized incidence ratio (SIR) being 1.32 among men and 1.66 among women. In women, the risk of cancer of the rectum was significantly increased (SIR 1.42), whereas the risks of cancer of the corpus uteri and multiple myeloma were lower than expected (SIR 0.76 and 0.53, respectively). In men, the incidence of cancer of the larynx was significantly reduced (SIR 0.63) and that of the bladder increased (SIR 1.25). When both sexes were combined, cancers of the colon (SIR 1.17) and rectum (SIR 1.28) also showed a significantly elevated risk. A reduction in risk was seen in stomach cancer (SIR 0.88) and lymphatic leukaemia (SIR 0.55). The increased lung cancer risk may be due to local inflammatory changes. It is possible that differences in the immune system, e.g. natural killer cell activity, explain some of the reduced cancer risks.