Nosocomial respiratory tract infection with Acinetobacter calcoaceticus occurs frequently in many hospitals. An outbreak of respiratory tract infections in an intensive care unit provided an opportunity to study clinical and epidemiologic characteristics of such infections. Retrospective studies demonstrated that A. calcoaceticus in sputum was significantly associated with endotracheal intubation (p = 0.03) and continuous positive pressure ventilation (p less than 0.02). After control measures had interrupted the outbreak, a prospective microbiologic investigation demonstrated that one third of the hospital personnel had transient hand colonization with multiple strains of A. calcoaceticus. Pharyngeal, vaginal and rectal carriage was rare. A pulmonary therapist with chronic dermatitis had persistent hand colonization with the epidemic strain, and he contaminated respiratory therapy equipment. Cross contamination of respiratory therapy equipment occurred while in use, but no other inanimate reservoir was demonstrated. Although previous studies have implied that the inanimate hospital environment has unique reservoirs of A. calcoaceticus, these reservoirs were not implicated in this outbreak. Human skin must be considered an important reservoir of A. calcoaceticus.