A rapid, inexpensive, and reliable antimycobacterial susceptibility assay is needed to effectively deal with the increasing prevalence of resistant strains of M. tuberculosis in Thailand. The microtiter plate Alamar blue assay (MABA) appears to be promising but requires additional data regarding correlation with clinical outcome. The correlation between the susceptibility by MABA and clinical outcomes was studied. There were 123 specimens obtained from extra-pulmonary sites of 108 patients at Ramathibodi Hospital in 1999. The authors found that susceptibility of M. tuberculosis isolates by the MABA correlated with the clinical outcome; patients with isolates sensitive to isoniazid, rifampicin, and ethambutol had a better clinical outcome than patients with isolates resistant to at least one of these drugs (p=0.004). Studies to determine this correlation in pulmonary tuberculosis are still needed.