Some epidemiological and clinical characteristics ofPityriasis rosea Gibert has led us to hypothesize that this disease may be the clinical manifestation of an infection caused by legionellas. We have thus tested the sera of 36 patients ill withPityriasis rosea and 19 controls forLegionella pneumophila serogroup 1–6 andLegionella micdadei antibodies. These, who had the same age and sex distribution as study patients, were receiving treatment for other diseases in the same ward. Also tested were 200 sera from the voluntary blood donors from the same region as study patients.Legionella micdadei antibodies were detected in 12 (33.3%)Pityriasis rosea cases and in one (5.2%) control. They were significantly more common in Pityriasis rosea cases than in either controls or voluntary blood donor population. The findings to date encourage continued research into the causative relationship between theLegionella micddadei infection and the onset ofPityriasis rosea Gibert.