Coal tar, which is widely used in the treatment of patients with atopic dermatitis, chronic eczema, and psoriasis, contains a large amount of polycyclic aromatic hydrocarbons (PAH). Some of the PAH compounds are known either to be carcinogenic or to potentiate the effects of other carcinogenic substances. In the present study, the excretion patterns of 1-hydroxypyrene (metabolite of pyrene) and alpha-naphthol (metabolite of naphthalene) in urine were studied in 2 patients, both treated once a day with coal tar pitch covering more than 50% of the skin. After 1 week of treatment, the concentration of both alpha-naphthol and 1-hydroxypyrene increased approximately 100 times. However, the concentration after 3 weeks of treatment was decreased to approximately the concentration measured before initiation of the treatment, even though the patients were coal tar-treated with unchanged intensity. The measured concentrations of alpha-naphthol and 1-hydroxypyrene in the urine of the 2 patients exceeded by order of magnitude the levels measured in the urine of occupationally exposed workers, and in view of the present study, epidemiological studies are needed to clarify to what extent coal tar treatment results in an increased risk of skin cancer, and e.g. bladder cancer.