OBJECTIVE: Studies have identified two types of antidepressant response: true drug response and placebo pattern response. This study examined the relationship between true drug response and choline-creatine ratios in the basal ganglia of depressed patients treated with fluoxetine. METHOD: The authors evaluated drug-free outpatients with major depression before (N=41) and after (N=15) 8 weeks of fluoxetine treatment, 20 mg/day, by using proton magnetic resonance spectroscopy. RESULTS: There was a significant difference in the degree of change from baseline to week 8 in choline-creatine ratios between the true drug response group (N=8) and the placebo pattern response/nonresponse group (N=7); the true drug response patients had a 20% increase in choline-creatine ratios, and the placebo pattern response/nonresponse patients had a 12% decrease in choline-creatine ratios. CONCLUSIONS: These data suggest that true drug response to fluoxetine treatment in depression may be associated with an increase in choline-creatine ratios in the basal ganglia.