A diffuse pneumonia characterized by short Giemsa-stained, gram-negative rods that were not cultured was diagnosed at autopsy in the fall of 1975 in three immunosuppressed patients who had had renal transplants. An acute fibrinous exudate was found, with macrophages, neutrophils, and numerous short bacilli in each inflamed alveolus as shown by Giemsa's stain (Wohlbach modification). Transplantation was stopped for 1 month. The institutional infection control committee discussed the cases, and increased observation. After the Legionnaires' disease agent was identified in 1977, the bacteria were stained by Dieterle's method and identified as Legionnaires' disease bacteria by direct fluorescent antibody test. No further cases have been identified in the succeeding 62 renal transplantations at this medical center. Although Wohlbach's modification of Giemsa has given consistently good staining of the Legionnaires' disease bacterium in our cases, unclarified factors may interfere with good staining, and the May-Grünwald Giemsa stain was unsatisfactory.