BACKGROUND Only a few cases of nosocomial Legionella sp. infection have been reported in children. We report the clinical and epidemiologic data of five nosocomial legionellosis cases that occurred in the Pediatric Nephrology Service between August, 1994, and December, 1998, and the control measures adopted. METHODS The Hospital Materno-Infantil Vall d'Hebron, Barcelona, is a 407-bed tertiary care hospital. The pediatric kidney transplant unit has three isolated beds in the same ward within the Pediatric Nephrology Service. Diagnostic workup to establish Legionella pneumophila infection included culture, fluorescent antibody and serologic studies. Macrorestriction analysis of genomic DNA was used as epidemiologic markers of the isolated strains. RESULTS In May, 1996, a case of L. pneumophila serogroup 6 pneumonia was identified in a 19-year-old youth who had received a kidney transplant 16 days earlier. Retrospective and prospective analysis of legionellosis cases diagnosed at our center up to August, 1994, yielded four additional cases. Four patients had had a kidney transplant and were receiving immunosuppressive therapy, and the fifth had been diagnosed with systemic lupus erythematosus with renal involvement. L. pneumophila serogroup 6 was isolated in bronchial secretions in four cases; in the fifth patient the diagnosis was made by serology. L. pneumophila serogroup 6 was isolated from potable water of the hospital. Molecular epidemiologic methods revealed the identity of the environmental and clinical isolates. Showering was implicated as the most feasible means of exposure to contaminated water. CONCLUSIONS Nosocomial legionellosis, albeit rare in children, should be considered in the differential diagnosis of pneumonias, particularly in immunosuppressed children, because the fatality rate may be high without early diagnosis and treatment.