Nontuberculous mycobacterial infections are becoming more common. Recently, Mycobacterium fortuitum and other rapidly growing mycobacteria have been found to cause severe skin and soft-tissue infections in association with nail salon whirlpool footbaths. We recently investigated a large outbreak of M. fortuitum furunculosis among women who received pedicures at a single nail salon. To better define the clinical course of such infections, we collected clinical details from physicians who were treating outbreak patients. We constructed multivariable linear models to evaluate the effect of antibiotic treatment on disease duration. Sixty-one patients were included in the investigation. The mean disease duration was 170 days (range, 41-336 days). Forty-eight persons received antibiotic therapy for a median period of 4 months (range, 1-6 months), and 13 persons were untreated. Isolates were most susceptible to ciprofloxacin and minocycline. Early administration of therapy was associated with shorter duration of disease only in persons with multiple boils (P<.01). One untreated, healthy patient had lymphatic disease dissemination.