The purpose of the present paper was to investigate the efficacy of augmentation of clomipramine (CMI) by tandospirone in 36 untreated outpatients with major depressive disorder. Twelve patients were treated with CMI and tandospirone (T group), 12 with CMI and diazepam (D group) and 12 with CMI alone (C group) for 6 weeks. No statistically significant differences in the percentage improvement of the Hamilton Depression Rating Scale (17 items; HDRS-17) and the Hamilton Anxiety Rating Scale (14 items; HARS-14) scores were shown among the three treatment groups. However, at 2 weeks, the percentage improvement of HDRS-17 score in the T group tended to be higher than that in the D and C groups, although there was no statistically significant difference among the three treatment groups. No change in plasma prolactin level or adverse events was induced by the addition of tandospirone. These results suggest that 6 weeks of treatment with tandospirone or diazepam was not effective for augmentation of CMI in major depressive disorder patients. However, augmentation of antidepressants by tandospirone administration for a few weeks might induce early expression of antidepressive effects.