The aim of the study was to ascertain whether there is an increased occurrence of proteinuria and/or haematuria among lung cancer patients. As a control group we selected patients with bronchial asthma treated at the same hospital and during the same period as the lung cancer patients. The study comprised a retrospective part, where observations were made at two hospitals on 166 and 284 lung cancer patients. Proteinuria was observed in 9% and 13% and haematuria in 5% and 9% of these patients. The prevalences were significantly greater than among the asthmatic patients. This result prompted a prospective case-control study involving 150 consecutive patients with lung cancer and asthma, respectively. Twelve patients in the cancer group and one asthmatic patient had a daily urine protein excretion exceeding 100 mg (P less than 0.004); haematuria was noted in 14 and one, respectively (P less than 0.002). We conclude that the occurrence of proteinuria and haematuria is unexpectedly increased among patients with lung cancer.