To date, it is unknown whether waist circumference can be measured validly and reliably when a subject is in a supine position. This issue is relevant when international standards for healthy participants are applied to persons with severe intellectual, sensory, and motor disabilities. Thus, the aims of our study were (1) to determine the validity of waist circumference measurements obtained in a supine position, (2) to formulate an equation that predicts standing waist circumference from measurements obtained in a supine position, and (3) to determine the reliability of measuring waist circumference in persons with severe intellectual, sensory, and motor disabilities. First, we performed a validity study in 160 healthy participants, in which we compared waist circumference obtained in standing and supine positions. We also conducted a test-retest study in 43 participants with severe intellectual, sensory, and motor disabilities, in which we measured the waist circumference with participants in the supine position. Validity was assessed with paired t-test and Wilcoxon signed rank test. A prediction equation was estimated with multiple regression analysis. Reliability was assessed by Wilcoxon signed rank test, limits of agreement (LOA), and intraclass correlation coefficients (ICC). Paired t-test and Wilcoxon signed rank test revealed significant differences between standing and supine waist circumference measurements. We formulated an equation to predict waist circumference (R(2) = 0.964, p <0.001). There were no significant differences between test and retest waist circumference values in disabled participants (p = 0.208; Wilcoxon signed rank test). The LOA was 6.36 cm, indicating a considerable natural variation at the individual level. ICC was .98 (p <0.001). We found that the validity of supine waist circumference is biased towards higher values (1.5 cm) of standing waist circumference. However, standing waist circumference can be predicted from supine measurements using a simple prediction equation. This equation allows the comparison of supine measurements of disabled persons with the international standards. Supine waist circumference can be reliably measured in participants with severe intellectual, sensory, and motor disabilities. (C) 2010 Elsevier Ltd. All rights reserved.