The aim of the present study was to investigate the usefulness of abdominal diameters to indicate visceral fat, their relationship with serum lipids and their capability of detecting changes in visceral fat. Before and after weight loss, visceral and subcutaneous fat, and the sagittal and transverse diameters were assessed by magnetic resonance imaging (MRI) in forty-seven obese men and forty-seven premenopausal obese women with an initial body mass index of 31·0 (SD2·4) kg/m². In a subsample (n21), diameters, were also measured by anthropometry in the standing and supine positions. They were strongly correlated with the diameters derived from the MRI scans. Serum levels of total and HDL-cholesterol and triacylglycerol were measured before weight loss. In women the sagittal diameter correlated less strongly with visceral fat than anthropometrically-assessed waist circumference and waist:hip ratio (WHR). In men these associations were comparable. Changes in visceral fat with weight loss were more strongly correlated with changes in the sagittal diameter and sagittal:transverse diameter ratio (STR) than with changes in waist circumference or WHR in men. In women, changes in the anthropometric variables and the separate diameters (except STR) were not associated with visceral fat loss. In men, but not in women, both the sagittal diameter and the visceral fat area were related to serum lipids. It is concluded that the sagittal diameter and STR may have advantages over waist circumference and WHR in men, particularly in assessing changes in visceral fat, but this could not be demonstrated in women. The ability to predict visceral fat from circumferences and diameters or their ratios is, however, limited in obese men and women.