Objective: To investigate the effect of a breakfast meal on bioelectrical impedance (BI). Design: Three separate interventions. Setting: A university based study. Subjects: Young, healthy volunteers recruited from staff. Twenty-nine subjects (11 men, 18 women), ten subjects (two men and eight women) and 13 subjects (2 men and 11 women) completed the first, second and third protocol, respectively. Interventions: Total body BI (protocol 1) or both total body BI and segmental BI (namely arm, leg or torso BI); (protocol 3) was measured in the fasting state and for up to 5 h after the consumption of a breakfast meal containing 28% energy from fat. In the second protocol, total body BI was measured in the same way on two occasions after subjects consumed isocaloric meals containing either 28% energy or 4% energy from fat. Results: Consumption of a 2300 kJ meal was followed by a significant (P=0.0002) decrease in BI (95% confidence intervals 12.5 and 35.3), a change which occurred 2 h after the meal and continued until 5 h, irrespective of meal fat content. The fall in total body BI was accounted for primarily by a fall in the BI of the limbs, with virtually no contribution from the torso. Conclusion: To ensure consistency in the interpretation of BI for body composition analysis, it is important that measurements are made in the fasting state. Sponsorship: MRG received a Deakin University Postgraduate Award.