Current concern over the carcinogenic potential of cytotoxic drugs and photochemotherapy used in the management of psoriasis increases the need for an accurate assessment of the 'background' incidence of carcinoma in psoriatics not receiving these modalities. This study compares the observed incidence of carcinoma in 719 tar-treated psoriatics with that expected, based on data from the West of Scotland Cancer Registry. No increase in malignancy was found in psoriasis over that expected in the general population.