In each of 14 patients, two small but comparable psoriatic lesions were infused for 2 weeks with either saline or a saline solution of peptide T (as its analog D-ala¹-peptide T amide) at 10^-7 mol/L with Alzet osmotic pumps worn extracorporeally. During infusion, lesions were photographed and scored for clinical features of psoriasis on a 9-point scale. After another 7 days, biopsy specimens were taken from the infused sites, and sections were scored for features of psoriasis on a 19-point scale. The differences between means for data from saline and peptide T-infused lesions were evaluated statistically. Peptide T-infused lesions improved clinically; scores decreased from a mean of 4.35 initially to 1.57 at biopsy, whereas control lesions changed from 4.43 to 3.57 (p < 0.01 for 1.57 vs 3.57). Histologic scores were also significantly different (5.28 for peptide T vs 10.00 for controls, 0.05 < p < 0.02). This study provides evidence that intralesionally infused peptide T demonstrates some clearing effect in psoriasis.