Plasma levels of gamma-aminobutyric acid (GABA) were determined in 68 healthy controls and in 133 patients with mood disorder. Plasma GABA levels were significantly lower in the patients with mood disorder compared to controls. Levels of plasma GABA were similar among diagnostic groups (primary unipolar depression, bipolar depression, mania, and secondary depression). No differences in plasma GABA were found in patients classified according to family history, nor were any correlations found between plasma GABA levels and severity of depression as determined by the 17-item Hamilton Rating Scale for Depression. These findings support the notion that low plasma GABA may represent a biological marker for mood disorder.