The measurement of the body’s carbon (C), oxygen (O), hydrogen (H), and nitrogen (N) content can be used to calculate the relative amounts of fat, protein, and water. A system based on prompt-gamma neutron activation analysis (PGNAA), coupled with the associated particle imaging (API) technique, is being developed for this purpose. A compact D,T neutron generator (~10 7 n/s output) with an internal alpha-particle detector is used. The counting system consists of 6 shielded bismuth germanate (BGO) detectors (10.2 cmu10.2 cmu10.2 cm) operated with fast-timing electronics to detect only gamma-rays within a 100 ns time window following a trigger pulse generated by the alpha detector. The body can be scanned from the shoulders to the knees within about 30 min, with the equivalent whole-body dose <0.4 mSv. The cumulative gamma-ray spectra in the 2 MeV to 8 MeV region is collected and analyzed for multiple peaks attributed to body C,O,H, and N. Measurement precision for each element, based on tissue-equivalent phantoms, are in the 2–5% range, which are sufficient for population studies in adults. Further improvements are needed to extend the measurements to pediatric clinical research studies.