BACKGROUND: Rapidly growing mycobacteria (RGM) are ubiquitous in the environment such as water and soil. Recently, infections by RGM are described in increasing frequency, but isolation from blood cultures has been rarely reported in Korea. The aim of this study was to identify blood culture isolates by 16S rDNA sequencing and to determine their clinical significance. METHODS: Blood cultures were performed using an automated blood culture instrument (BacT/ Alert 3D, Organon Teknika, Durham, USA), and the isolates were identified by nucleotide sequencing for the 16S rDNA after PCR amplification. The clinical significance was determined by patient's symptoms, laboratory findings, radiologic findings, underlying diseases, clinical course, and response to therapy etc. RESULTS: RGM were isolated from 16 patients: the organisms were identified as Mycobacterium mucogenicum in 12 patients, and M. cosmeticum in 4 patients. Ten isolates were considered clinically significant and the remaining four as contaminants. CONCLUSIONS: RGM can be detected in routine blood cultures; therefore, they should not be confused with other gram-positive bacilli such as Corynebacteria, and the possibility of RGM in blood stream infections must be considered. It is necessary to identify RGM up to the species level for proper diagnosis and treatment. The clinical significance of RGM from blood cultures must be carefully assessed with the consideration of contamination.