Objective: To evaluate the utility of weight-for-length (defined as gm/cm(3), known as the "ponderal index") as a complementary measure of growth in infants in neonatal intensive care units (NICUs)., Study Design: This was a secondary analysis of infants (n=1214) of gestational age 26 to 29 weeks at birth, included in a registry database (1991-2003), who had growth data at birth and discharge. Weight-for-age and weight-for-length were categorized as small (<10th percentile), appropriate, or large (>90th percentile)., Results: Statistical agreement between the weight-for-age and weight-for-length measures was poor (kappa=0.02 at birth, 0.10 at discharge; Bowker test for symmetry, P< .0001). From birth to discharge, the percentage of small-for-age infants increased from 12% to 21%, the percentage of small-for-length infants decreased from 10% to 4%, the percentage of large-for-age infants remained similar (<1%), and the percentage of large-for-length infants increased from 5% to 17%. At discharge, 92% of the small-for-age infants were appropriate or large-for-length, and 19% of the appropriate-for-age infants were large-for-length., Conclusions: Weight-for-age and weight-for-length are complementary measures. Weight-for-length or other measures of body proportionality should be considered for inclusion in routine growth monitoring of infants in the NICU.