BACKGROUND: It is recommended that all rapidly growing mycobacteria (RGM) isolated from patients with mycobacteriosis are subjected to antimicrobial susceptibility testing. The current study was aimed to perform susceptibility test on clinical strains of RGM isolated from patients with mycobacteriosis and to determine the clinical significance of the isolates. METHODS: For 17 patients with RGM infection from 2002 to 2006 at Ulsan University Hospital, medical records were reviewed retrospectively and anti-mycobacterial susceptibility test was performed for the clinical isolates by broth microdilution method. RESULTS: Rates of susceptible strains of RGMs against individual drugs were as follows: amikacin 100%, cefoxitin 59%, ciprofloxacin 82%, clarithromycin 71%, doxycycline 18%, imipenem 91% (M. fortuitum), sulfamethoxazole 71%, and tobramycin 100% (M. chelonae). Ten of the 17 nontuberculous mycobacteria (NTM) patients had been treated with anti-tuberculosis drugs initially. Anti-tuberculosis drugs were continued in 3 patients and changed to other antimicrobial agents effective to NTM in 4 patients, all of whom were cured. Five of 7 NTM patients who had been treated with anti-NTM treatment were cured. All isolates from the patients treated with anti-NTM drugs were susceptible to at least one of the drugs administered. CONCLUSION: Clinical isolates of RGMs showed fully susceptible to amikacin, while highly resistant to doxycycline and variable to other drugs depending on the species.