A cluster of five cases of Legionnaires' disease in renal transplant patients is described. They were treated with erythromycin and rifampicin, and all five survived. Two of them had rejected their grafts prior to theirLegionellapneumonia; two rejected their transplants after reduction of immunosuppressive therapy to combat the infection. L pneumophilawas present in the water distribution system of the hospital. Eradication measures included flushing the water pipes to the transplantation ward with hot and hyperchlorinated water, raising the warm water temperature to 60°C, and installing ultraviolet (UV) irradiation units on the warm and cold water pipes to the ward. These measures were successful in that no new cases of legionellosis occurred afterwards.L pneumophilacould subsequently not be demonstrated by culture in plastic shower hoses supplied with UV-irradiated water.L pneumophilacould be demonstrated by direct fluorescent antibody technique, but nonspecific reactions cannot be excluded. A higher prevalence of elevatedL pneumophilaantibody titers was observed in patients nursed for more than four weeks in the hospital than in patients with a shorter hospital stay, in hospital staff members, or in the general population. It seems that, with appropriate control measures, transplantation activities need not be discontinued in the presence of a minor cluster of Legionnaires' disease in renal transplant patients.