Mycobacterium avium-intracellulare complex (MAC) has become one of major human pathogens, however, its routes of transmission and environmental reservoirs causing human infection were not yet elucidated. We reported three families affected by pulmonary Mycobacterium avium (M. avium) disease. Previous reports on MAC diseases observed in the same family were very rare. The purposes of this study were to investigate whether the infected M. avium was the same strain among cases in the same family and to examine the possibility of human-to-human transmission, or infection from exposure to a common environmental reservoir.M. avium isolates from nine cases of three families were examined by DNA polymorphism based typing technique, restriction fragment length polymorphism (RFLP) analysis using insertion sequence IS1245 as a probe, to type the strains. Some isolates were subcultured to a single clone.All strains isolated from cases in the same family showed different patterns by the RFLP analysis. And not only simultaneous polyclonal infection but also repeated polyclonal infections were observed in some patients.The results suggest importance of underlying anti-mycobacterial immunological impairment and defects of local defense rather than virulence of infected strains as the pathogenesis of pulmonary M. avium disease.