Different subtypes of ovarian cancer appear to have different causes; however, the association between body mass index (BMI) and the different subtypes is unclear. We examined the associations between body-mass index (BMI) and weight gain and risk of the different histological subtypes of epithelial ovarian cancer in a case-control study in Australia. Cases aged 18-79 with a new diagnosis of invasive epithelial ovarian cancer (n = 1,269) or borderline tumor (n = 311) were identified through a network of clinics and cancer registries throughout Australia. Controls (n = 1,509) were selected from the Electoral Roll. Height and weight (1 year previously, at age 20 and maximum weight) and other risk factor information were ascertained via a self-administered questionnaire. Obesity was positively associated with clear cell tumors (Odds Ratio 2.3; 95% confidence interval 1.2-4.2) but not invasive endometrioid or mucinous tumors. Although there was no association with invasive serous tumors overall (0.9; 0.7-1.2), we did see an increased risk of serous peritoneal tumors (2.9; 1.7-4.9), but not of serous tumors of the ovary and fallopian tube. Of the borderline subtypes, obesity was positively associated with serous (1.8; 1.1-2.8) but not mucinous tumors (1.1; 0.7-1.7). Overweight was not associated with any subtype overall. There was no association with BMI at age 20, or weight gain for any of the histological subtypes. These results add to the current evidence that obesity increases a woman's risk of developing distinct histological subtypes of ovarian cancer.