Weight gain is an important consequence of the intensive treatment of insulin-dependent diabetes mellitus (IDDM). Despite this, body composition in this disorder has not been well studied. Bioelectrical impedance analysis (BIA) has been shown to be a reliable, noninvasive method of assessing body composition but little is known about its use in IDDM. Therefore, a regression model estimating fat-free volume mass from bioelectrical impedance measurements obtained with proximal electrode placements was developed in 46 adults with IDDM through use of dual-energy X-ray absorptiometry (DXA) for reference values of fat-free mass. No effect of blood glucose concentration or prandial state on the impedance characteristics was observed. A regression model was developed incorporating the ratio of height squared to the minimum resistance of four limb-lead combinations (H2/R), total body weight, and a weight-sex interaction. With this model, BIA correlated closely with DXA (R2 = 0.982, residual SD = 1.3 kg). The applicability of these equations to non-insulin-dependent diabetes mellitus is unknown.