Summary Background Delineating a new exanthem is difficult. Atypical or modified types of already known conditions can easily be mistaken for new entities. Objectives To report 29 cases of an acute, self-limiting exanthem never described before. Methods Clinical description, serology, polymerase chain reaction (PCR) and other laboratory investigations. Results The eruption starts with bright pink-red slightly oedematous macules symmetrically arranged on the trunk. The lesions tend to coalesce, and spread to the limbs on which they may acquire a marginated annular appearance. The lesions invariably spare the face (including oral mucous membranes), palms and soles. The eruption is pruritic and lasts 20–30 days, clearing spontaneously. The disease has a predilection for men aged 17–56 years and occurs mostly between spring and summer. Serological tests, PCR assays and investigations for bacteria, parasites or viruses were consistently negative. Conclusions The clinical picture does not recall any of the classical exanthems or their atypical forms.