The first case of Legionnaires' disease recognized in Pennsylvania since the Philadelphia epidemic of 1976 was that of a 53-year-old emphysematous man who had extensive unilateral pneumonia accompanied by high fever, hypoxemia, and disorientation. His illness progressed despite treatment with cephalothin, but he recovered coincident with the administration of gentamicin and erythromycin. The diagnosis was established serologically by a 32-fold rise in antibody titer to the agent of Legionnaires' disease. Similar illness did not affect others in his community, and infection in four family members was excluded clinically and serologically.