Recently the pharmacologic role of leukotrienes (LTs), especially that of LTB4, has been intensively investigated in psoriasis. In vitro, R 68 151 is a potent 5-lipoxygenase inhibitor and consequently reduces LTB4 formation. Therefore the role of an in vitro 5-lipoxygenase inhibitor and its clinical use in psoriasis were evaluated with topical R 68 151 in a double-blind vehicle-controlled study. Eighty-eight patients with localized psoriatic lesions were treated twice daily with R 68 151 2% ointment (n = 44) or vehicle ointment (n = 44) during 4 weeks. Most patients (n = 73) had psoriasis vulgaris (n = 37, R 68 151; n = 36, vehicle). In 27% of the R 68 151-treated patients with psoriasis vulgaris, the lesions disappeared or showed marked improvement, compared with 8% in the vehicle group (X2, p = 0.06). The scores in global evaluation, however, were significantly different between both treatment groups (p less than 0.05, Mann-Whitney U test). The improvement of the mean symptom score with R 68 151 was 46% for scaling and 34% for erythema at the end of the study compared with an improvement of 6% and a deterioration of 3%, respectively, in the control group (p less than 0.05, p less than 0.01; Mann-Whitney U test). The global evaluation in the total group of patients with psoriasis (all different subtypes) was consistent with the response rate in the group of patients with psoriasis vulgaris: 30% in the test group versus 11% in the control group (X2, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)