BACKGROUND Kawasaki's disease may have numerous atypical forms and these must be recognized in order to avoid delay of treatment. We report a case of psoriasis, first pustular and then guttate, occurring during Kawasaki's disease, and discuss a common pathophysiological mechanism. METHODS A 3 year-old boy was seen for a febrile exanthema suggestive of Kawasaki disease (bilateral conjunctivitis, red and fissured lips, palmoplantar erythema, scarlet fever-like rash and perineal desquamation) associated with pustular lesions. A biopsy specimen of a pustular area showed histological features consistent with the diagnosis of pustular psoriasis. No coronary abnormality was found. The child was treated with intravenous immunoglobulins (2 g/kg) and oral aspirin (60 mg/kg/d). All the symptoms disappeared and immediate follow-up was marked by the appearance of guttate psoriasis. CONCLUSIONS Onset of psoriatic lesions during Kawasaki disease has been reported in 12 cases, either in acute phase or in immediate follow-up. Coronary complications have been found in 4 of 5 cases with acute psoriasis, suggesting a severe prognosis for this association. The hypothesis of a common pathophysiological mechanism is discussed with the intervention of a bacterial toxin acting as a superantigen and resulting in an strong activation of T-cells that leads to keratinocyte activation. The psoriatic lesions could hence be considered as a form of Köbner's phenomenon.