OBJECTIVE Metabolic abnormalities in patients with bipolar disorder may be secondary to obesity, aspects of the disorder or its treatment. To investigate this further, the purpose the present study was to compare insulin resistance, components of the metabolic syndrome and adiponectin levels in a group of overweight bipolar patients taking sodium valproate and a group of non-psychiatric control subjects. METHODS Data were collected from 60 overweight bipolar patients who had experienced clinically significant weight gain thought to be related to sodium valproate treatment and from 60 control subjects without psychiatric illness matched for age, gender, body mass index and ethnicity. RESULTS The frequency of the metabolic syndrome was high in both groups (50% and 32%, respectively), although not significantly different between groups (p = 0.06). Similar frequencies of insulin resistance (HOMA-IR), abdominal obesity, hypertriglyceridaemia, hypertension and fasting hyperglycaemia were found in both groups. High-density lipoprotein cholesterol levels were lower in patients (p = 0.006), while adiponectin was unexpectedly higher than in control subjects (9.6+/-5.9 microg mL(-1) vs 7.4+/-4.3 microg mL(-1), p = 0.03). The frequencies of insulin resistance (HOMA-IR), the metabolic syndrome and its individual components were not significantly different in patients taking atypical antipsychotic medication and patients not on these medications. CONCLUSIONS Frequencies of insulin resistance and the metabolic syndrome were similar in bipolar patients taking sodium valproate and matched control subjects, but dyslipidaemia was more frequent. Adiponectin levels were higher in patients. Further research is required to clarify the reasons for these findings.