One hundred twenty-five cases of disease due to rapidly growing mycobacteria were observed over a four-year period. Cutaneous infections accounted for 74 cases (59%). Of these, 40 followed surgical procedures (especially augmentation mammaplasty or median sternotomy), and 34 were due to accidental penetrating trauma. Among the 24 patients with pulmonary disease, the mean age was approximately 60 years, the majority of patients (63%) were women, and most had unilateral noncavitary disease. Other infections included disseminated disease with multiple nodular skin lesions and positive blood cultures, cervical lymphadenitis, keratitis, and endocarditis associated with a prosthetic valve. Infected tissues showed mixed acute and granulomatous inflammation; acid-fast bacilli, when present, occurred in extracellular clumps within microabscesses. Mycobacterium fortuitum and Mycobacterium chelonei were encountered with approximately equal frequency; 80% of isolates of M. chelonei were subspecies abscessus, and 83% of isolates of M. fortuitum were biovariant fortuitum. The outcome in these infections was generally good, although 9% of the patients, including all those with endocarditis, died. Infections due to M. fortuitum and M. chelonei are probably markedly under-diagnosed, and these organisms are capable of causing a wide spectrum of clinical disease.