Infection Control surveillance revealed an abnormally high number of sputum samples growing Mycobacterium fortuitum from a single ward that houses the human immunodeficiency virus service. We investigated the outbreak using a retrospective case-control study and molecular epidemiology. A total of 47 patients were identified with >/=1 sputum sample that grew M. fortuitum. No significant demographic or clinical variables were found to be associated with colonization. Environmental investigation demonstrated that the M. fortuitum isolated from patients was identical to the ice machine isolates by pulsed-field gel electrophoresis. Fortunately, there has been no evidence of progressive pulmonary disease developing in our patients after >/=6 months of follow-up. All cases were thought to represent transient colonization and not infection. The ice machine was disconnected and cleaned successfully with vinegar and then bleach. Only when a 0.5-micrometer filter was installed were no further patients colonized or water cultures found to be positive.