Repetitive transcranial magnetic stimulation (rTMS) efficacy in the treatment of major depression has been shown in both low frequency right-sided and high frequency left-sided stimulation over the dorsolateral prefrontal cortex (DLPFC). The aim of the present investigation was to evaluate the hypothesis of an additive effect of bilateral stimulation compare to sequential to unilateral stimulation. Sixty patients with treatment-resistant depression were assigned to receive either low-frequency rTMS over the right DLPFC (140 s x 1 Hz) followed by controlateral sham (unilateral group, n=20), low frequency right DLPFC rTMS followed by left DLPFC high frequency rTMS (5 s x 10 Hz) (bilateral group, n=20), or bilateral sham (sham group, n=20) in a 3 weeks double-blind, randomized trial. The primary outcome variable was the score on Hamilton Depression Scale (HAM-D). Low frequency right-sided and sequential bilateral stimulation showed different antidepressant efficacy at 3 weeks and across the full duration of the study, only the unilateral method appearing significantly more effective than sham at the end of the trial, and correlated to the higher percent of remitters (30% of the group vs. 10% -bilateral- and 5% -sham). Unilateral stimulation, but not bilateral, showed higher antidepressant efficacy compared to sham stimulation. The data suggest that right-sided low frequency stimulation may be a first line treatment alternative in resistant depression. To confirm and extend these findings further studies require a longer follow-up period, supported by biological observation and replication.