BACKGROUND: Uncertainty exists regarding the best approach for treating bipolar depression among patients already receiving a first-line mood stabilizer. The aim of this pilot study was to compare adding a second mood stabilizer or an antidepressant at this treatment decision point. METHODS: Twelve-week, randomized, double-blind pilot trial comparing the addition of lamotrigine or citalopram for bipolar depressed patients on mood stabilizer medication. Change in depressive symptoms and risk of switch were examined. RESULTS: Twenty subjects were randomized. Each treatment group experienced a significant mean reduction in total MADRS scores (citalopram Delta - 14.2, p=0.002; lamotrigine Delta - 13.3, p= 0.001), and there was no significant difference between treatment groups (p=0.78). Total response rates increased from 31.6% at week 6 to 52.6% at week 12. One out of ten patients in each group experienced a switch to hypomania. LIMITATIONS: Small sample size. Lack of a placebo arm. CONCLUSIONS: Results of this small trial suggest that both lamotrigine and citalopram appear to be reasonable choices as add-on acute treatment for bipolar depression, with response rates continuing to rise considerably past 6 weeks of treatment.