Among 1,253 patients with psoriasis who were enrolled in a study of photochemotherapy, 33% had a history of a Koebner response (KR) at the site of physical trauma. The early onset of psoriasis and previous treatment with multiple types of therapy were each independently associated with the KR. Among patients with both onset before age 15 years and previous treatment with four or more types of therapy, 75% reported the KR, compared with only 5% of patients with onset after age 30 years and no previous therapy. At the time of enrollment in the photochemotherapy study, the history of the KR was not associated with worsening, the duration of psoriasis, or the extent of skin surface involved. Patients with the KR may manifest their disease earlier in life or may be more prone to flare after any type of treatment.