We describe familial cases of cutaneous infection caused by Mycobacterium avium. A 45-year-old father, his 14-year-old son and 11-year-old daughter, among five persons in a family, presented with a 2-month history of inflammatory subcutaneous nodules and ulcerations. Histology of skin biopsy specimens showed granulomatous inflammation, and mycobacterial colonies isolated from the skin of each patient were identified as M. avium by DNA hybridization analysis. The patients were all treated successfully with combined drug therapy consisting of rifampicin, isoniazid and clarithromycin. Their lesions were purely cutaneous M. avium infection, without any visceral involvement. Neither systemic disease nor immunological impairment was detected in the family. However, they all used a circulating, constantly heated bath water system. The bath water was continuously heated to about 40 degrees C without changing the water for a few months, and M. avium was isolated from the filter of the bath tub heating unit. It is considered that this unusual familial cluster of cutaneous M. avium infection in healthy persons may have resulted from the use of contaminated bath water.