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The purpose of the study was to examine the validity of hydrodensitometry in estimating fat free mass (FFM) of spinal injured (SI) subjects. Fifteen SI subjects (10 males, 5 females) and 11 able bodied (AB) subjects (6 males, 5 females) underwent hydrostatic weighing and determination of total body water (TBW) by ethanol dilution. The two groups did not differ in body density, percent body fat, fat free mass, or percent body water unless matched for body weight and sex. The two groups were different in residual lung volume, weight in water, and sum of 8 skinfolds. The correlation between FFM calculated from total body water and FFM calculated from densitometry was 0.951 for A B, and 0.705 for SI subjects, respectively. The SEE for SI subjects (5.95 kg) was twice that for AB subjects (2.81 kg). Sources of possible error are discussed. Errors in measurement of body density, and consequently, of FFM in SI subjects may be related to corrections for residual lung volume in hydrodensitometry. In addition, underlying assumptions regarding the constancy of the body compartments are probably violated in SI subjects. It was concluded that current equations used to derive body density from hydrodensitometry in able bodied persons are inappropriate for spinal injured persons.