We studied catch-up growth, muscle and fat accretion, and body proportionality at 4 and 12 months of age corrected for prematurity in 30 very low birth weight (VLBW) (less than 1500 gm), 30 low birth weight (LBW) (1500 to 2499 gm) and 30 normal birth weight (greater than or equal to 2500 gm) infants who required newborn intensive care. At 4 and 12 months, the VLBW infants had significantly lower mean weight and length (p less than 0.01), but not lower occipitofrontal circumference percentiles, than the LBW and normal birth weight groups, and showed no catch-up weight or length growth between 4 and 12 months. All three groups had significant increases in mean upper mid-arm circumferences, mid-arm muscle circumferences, and arm muscle areas between 4 and 12 months. Mean mid-arm muscle circumferences and arm muscle areas were similar among the three groups at 4 months but became significantly stratified by birth weight groups by 12 months, with VLBW infants having the lowest mean value. In contrast, analysis of fat stores by triceps skin-fold thickness and arm fat area demonstrated no significant increases in any group between 4 and 12 months, except for arm fat area in the LBW group. The VLBW infants had significantly less fat than normal birth weight infants at 4 and 12 months. All three groups had proportional growth at both visits, as assessed by mid-arm circumference/head circumference ratio and weight-length percentile for age. The VLBW infants were significantly lighter for their length than normal birth weight infants. We conclude that VLBW infants have no first-year catch-up growth, remaining smaller than higher birth weight infants, although appropriately proportional. Somatic growth during the first year is due more to muscle than to fat accretion, especially in VLBW infants.