A bilateral comparison study of twenty-two outpatients with psoriasis assessed clearing experience and remission time on halves of the body to which tar oil and oil vehicle (an emollient) were applied twice daily. In addition, both sides were treated three times a week with suberythemogenic doses of ultraviolet B radiation. Of the eighteen patients who complied with the protocol, fourteen (78%) had clearing. There was no significant difference between body halves with regard to the number of treatments to clearing, the dose of ultraviolet B to clearing, or the remission time (p > 0.2, all comparisons). Our data indicate with 95% confidence that using tar oil in conjunction with ultraviolet B phototherapy reduces the average total ultraviolet B dose required for clearing by a maximum of 9%, in comparison with using oil vehicle and ultraviolet B. These findings suggest that there is insufficient evidence for substantial clinical benefit to recommend tar oil use in conjunction with outpatient ultraviolet B phototherapy for psoriasis.