An inverse relation between contact allergy and autoimmune diseases is suggested from epidemiological studies. The aim of this study was to investigate susceptibility and reactivity in patients with psoriasis, patients with diabetes and healthy controls in an experimental sensitization study. We sensitized 68 adult individuals (23 patients with psoriasis, 22 patients with diabetes and 23 healthy controls) with diphenylcyclopropenone (DPCP) and assessed challenge responses with visual scoring and ultrasound. Skin biopsies from challenged skin were investigated for differences in down-regulatory mechanisms with immunohistochemistry and gene-expression profiles using microarray technology. The sensitization ratios were 26%, 36% and 65% for the psoriatic, diabetic and healthy groups, respectively. Logistic regression analysis gave an odds ratio (OR) for a patient with psoriasis or diabetes type I of being sensitized to 0·18 [95% confidence interval (CI): 0·039-0·85], P = 0·031 and 0·74 (95% CI: 0·548-1·008), P = 0·056, respectively. A high degree of forkhead box P3-positive (FoxP3(+) ) cells were found in biopsies of positively challenged reactions, but only limited numbers in negatively challenged reactions, with no difference among the groups. No specific mRNA expression was found in the challenged skin of negative elicitation reactions, also indicating no sign of active down-regulation. The study contibutes strongly to the evidence of a decreased susceptibility to develop contact allergy in individuals with autoimmune diseases such as psoriasis.