Information about body fatness is important during nutritional assessment of infants, but current methods to estimate body composition in vivo are often not applicable in infants. Therefore, a new method based on magnetic resonance imaging (MRI) was developed. This method, which can assess the volume and distribution of adipose tissue (AT) as well as total body fat, was applied in 11 healthy full-term infants. Their total body water was also estimated using the isotope dilution technique. Adipose tissue volume (ATV) was calculated from AT area in 16 images of the body taken by an MRI scanner (1.5 tesla). AT area was assessed using a computer program in which AT criteria was defined by the observer. ATV of the infants was therefore evaluated once by three observers and twice by a fourth observer. The different observers estimated total, s.c., and non-s.c. ATV with a precision that varied between 1.9 and 7.2%, 2.0 and 4.8%, and 4.2 and 40.7%, respectively. Variations during AT area calculations accounted for a large part of the imprecision when assessing total and s.c. ATV. The linear relationship between percent total body water and total ATV in relation to body weight was significant in all evaluations. Although average total ATV varied when estimated by the four observers, there was, within each evaluation, a fairly constant order between infants with respect to their ATV. It is concluded that the MRI procedure represents a useful possibility to assess body fatness in infants.