Background: Bipolar disorder (BD) has been associated with an increased prevalence of weight gain and abnormally elevated plasma homocysteine (Hcy) levels. However, the relationship between BMI and Hcy in BD patients has not been investigated. This study aimed to explore this relationship in Chinese patients with BD. Methods: Plasma Hcy levels, socio-demographic parameters, clinical and anthropometric data were collected from 195 BD inpatients and 84 healthy controls. The level of plasma Hcy was determined by high-performance liquid chromatography. Body mass index (BMI) was calculated by body weight divided by the square of the height. The participants were divided into a high BMI group and a low BMI group using 24 kg/m(2) as a threshold. Results: The prevalence of high BMI was slightly elevated in BD patients in comparison to healthy controls. Patients with elevated BMI had significantly higher Hcy levels than patients with low BMI. Hcy level was an independent contributor of the occurrence of high BMI in BD patients. The level of Hcy was positively correlated with BMI in BD patients. In addition, depressive episodes of BD were positively correlated with the prevalence of high BMI and married BD patients were more likely to have high BMI levels. Conclusions: There is a close relationship between BMI and plasma Hcy levels in patients with BD, suggesting that Hcy may be an important indicator for BD-induced weight gain. This finding provides a new avenue for weight management of BD patients and to help avoid the potential risk of cardiovascular diseases.