ObJective To study the hypothalamic-pituitary-adrenal (HPA) axis function with dexamethasone suppression test (DST) in inpatients with unipolar depression or bipolar disorder at different mood states. Methods DST was performed in 38 inpatients with unipolar depression and 63 with bipolar disorder ([19 with type Ⅰ, 44 with type Ⅱ], [33 with depressive episode, 18 with manic episode and 12 with combined episodes]). After 4 weeks' treatment, DST was performed again on 17 patients with unipolar depression and 35 with bipolar disorder to compare the negative suppression ratio. Results Before treatment, the negative suppression rate of DST was significantly different between unipolar depression (36.8%) and bipolar disorder (14.3%), type Ⅰ bipolar disorder (10.5%), type Ⅱ bipolar disorder (15.9%) or bipolar disorder with current depressive episode (15.2%) (P<0.05). However, no statistic differences were showed among type Ⅰ bipolar disorder and type Ⅱ bipolar disorder, depressive episode of bipolar disorder (15.2%), manic episode of bipolar disorder (16.7%) or combined episodes of bipolar disorder (11.1%) (P>0.05). After treatment, the same comparison was performed, but negative suppression rate of DST was not significantly different among all the groups (P>0.05). With the clinical improvement, negative suppression rate of DST decreased in patients with unipolar disorder;while no significant differences were found between pre-treatment and post-treatment in patients with both unipolar and bipolar disorders (P>0.05). Conclusion At the status of illness, the negative suppression rate of DST in the unipolar depression, being independent from the clinical subtypes, types of episode and severity of the illness in bipolar disorder, is much higher than that in the bipolar disorder.