We studied the response to a standard skin injury in the involved and uninvolved skin of twenty-four subjects with psoriasis to determine whether a relationship exists between disease activity and the Koebner reaction. We found that 25% of patients had a positive Koebner reaction and 67% had a positive 'reverse' Koebner reaction (psoriasis clearing following skin injury). If psoriasis occurred in one area of injury, all injured areas developed psoriasis, and 'all-or-none' phenomenon. If psoriasis cleared from the traumatized area, no psoriasis occurred in the uninvolved sites. The 'reverse' Koebner reaction and the Koebner reaction are thus mutually exclusive. Disease activity, by our criteria, did not predict a positive Koebner reaction, but a positive Koebner reaction did predict subsequent disease activity. These observations suggest that humoral factors govern the development of clearing or psoriasis after a standard injury.