A possible association between contact allergy and psoriasis was examined in two different Danish populations. The first was a clinic-based register linkage population (n = 15,641) including patients with psoriasis and patch-test data. The second was a population-based cross-sectional study population obtained by random sampling in 1990, 1998 and 2006 (n = 4,989). An inverse relationship was found between a psoriasis diagnosis and a positive patch test in both populations, even after adjusting for age and gender (r = 0.58 and r = 0.64 for clinic-based and population-based data, respectively; P < 0.001 and P = 0.038, respectively). These findings may be suggestive of opposing immunological mechanisms in psoriasis and contact ...