Pulmonary function tests, including the transfer factor for carbon monoxide of the lungs (T1CO) and pulmonary capillary blood volume (Vc), were measured in 43 patients with disseminated testicular carcinoma and in eight patients with disseminated malignant melanoma before the start of a bleomycin-containing cytostatic combination. Their relation to the development of bleomycin-induced pulmonary toxicity was evaluated. We found two subgroups of patients, one group with and one group without an abnormal pretreatment T1CO and Vc. In the latter group the risk of bleomycin-induced pulmonary toxicity is not increased. Reduction in bleomycin dose during treatment is not necessary. The hypothesis is formulated that tumor embolism could be the explanation for this phenomenon.