Psoriatic patients were examined for serologic evidence of streptococcal infections by using antideoxyribonuclease B (ADB), which has been shown to be a more sensitive screening tool than antistreptolysin O (ASO) for detecting such evidence in other clinical circumstances. Antistreptococcal antibody titers (ASO and ADB) of 71 patients with psoriasis vulgaris, 7 with guttate psoriasis, and 6 with erythrodermic psoriasis were compared with ASO and ADB antibody titers of 25 non-psoriatic dermatologic patients hospitalized at the same time and of an adult group that was used as a control for the test itself. Thirty-six of 71 patients with psoriasis vulgaris had elevated titers of ADB or ASO (41% vs. 15%). Compared with control values the ADB titers were significantly higher in patients with psoriasis vulgaris (P less than 0.02) and patients with guttate psoriasis (P less than 0.05). The ASO titers were significantly higher in patients with psoriasis vulgaris than in controls (P less than 0.04). Within the group with psoriasis vulgaris, ADB titers were significantly higher than ASO titers (P less than 0.01). Among patients with psoriasis vulgaris, histories of sudden flares of psoriasis and early recurrences of psoriasis after previous successful hospital treatment, family histories of psoriasis, and histories of potential streptococcal infections were more frequent in those with elevated antistreptococcal antibodies.