In an analysis of legionellosis at Presbyterian-University Hospital in Pittsburgh during the 28 months after the original outbreak of pneumonia caused by Legionella micdadei , 101 consecutive renal-allograft recipients were studied. The prevalence of elevated titers of antibody to Legionella pneumophila or L. micdadei among the patients at the time of transplantation (21.6%) was not significantly different from that among kidney donors (13.4%; P greater than .25). Seroconversion occurred in 21 (23.6%) of 89 recipients during the six months after transplantation. Seven recipients who seroconverted were diagnosed as having legionella pneumonia, eight had fever and pneumonitis attributed to other causes but compatible (on clinical grounds) with legionella infection, and six had no clinical evidence of infectious pneumonia. The interval from transplantation to onset of pneumonia was shorter for recipients with evidence of L. micdadei pneumonia than for patients with L. pneumophila pneumonia (P = .006); this finding suggested that the former disease occurred when the recipients were most immunocompromised. The original outbreak of L. micdadei pneumonia was found to have subsided nine months after the last formally recognized case, but infections with L. pneumophila continued to occur thereafter.