BACKGROUND: Guttate psoriasis has a well-known association with streptococcal throat infections but the effects of these infections in patients with chronic psoriasis remains to be evaluated in a prospective study. OBJECTIVES: To determine whether streptococcal throat infections are more common in and can cause exacerbation in patients with chronic psoriasis. METHODS: Two hundred and eight patients with chronic plaque psoriasis and 116 unrelated age-matched household controls were followed for 1 year. At recruitment all patients were examined, their disease severity scored and throat swabs taken. Patients and corresponding controls were then re-examined and tested for streptococcal colonization whenever they reported sore throat or exacerbation of their psoriasis during the study period. RESULTS: The psoriasis patients reported sore throat significantly more often than controls (61 of 208 vs. three of 116, P