Abstract Sixteen patients with nosocomial Legionella micdadei pneumonia, diagnosed between 1977 and 1988, were studied retrospectively to define clinical and epidemiological characteristics of the disease. Also, a case-control study was performed comparing the five patients with L. micdadei pneumonia during a cluster of cases in 1982, with uninfected patients with the same underlying diagnoses. No significant differences were noted in the case-control study with regard to age, presence of leucopenia, intensity or duration of immunosuppressive therapy, bed location, duration of hospital stay, frequency of transplant rejection or overall mortality. Legionella micdadei isolates from a sink on the renal transport ward, from hot water storage tanks, and one clinical isolate had identical cellular fatty acid composition. Extensive sampling of other potential sources failed to yield the organism. This indirect evidence suggests potable water as the source of infection.