We studied the superoxide anion activity of psoriatic and normal dermal fibroblasts in culture. Superoxide anion production was measured by the technique of Pick and Mizel in fibroblasts cultured from biopsies of involved and uninvolved skin of patients with psoriasis and from skin biopsies of normal controls. Our results show that the liberation of superoxide anion was sig nificantly increased (p < 0.001) in fibroblasts obtained from involved areas of skin (5.58 nmol O2-/106cells/50 min) and also from uninvolved skin (p < 0.01) (4.60 nmol O2-/106 cells/50 min) of psoriatic patients, compared to controls (2.25 nmol O2-/106 cells/50 min). The increase in superoxide anion liberation was 100 and 150% in uninvolved and involved psoriatic fibroblasts, respectively. This suggests an important role for dermal fibroblasts in the inflammatory mechanism of psoriasis.