To assess the functional ability in low birth weight children at age 4-5, 114 survivors with very low birth weight (VLBW) less than or equal to 1,500 g, 166 survivors with birth weight 1,501-2,300g (LBW), and 115 comparison children with normal birth weight (NBW) were enrolled in a follow-up study. Twenty-four (21%) VLBW and 11 (6.6%) LBW-children had major clinical abnormalities compared to 1 (0.9%) of the NBW children. Twenty-two percent of the VLBW-children were below the 3rd-centile for height and weight. Fewer VLBW than LBW-children were neurologically and ophthalmologically normal. Even after exclusion of the handicapped children, the LBW as well as the VLBW children scored significantly lower than the NBW children in the McCarthy Scales of Children's Abilities, most markedly in the motor and perceptual performance scales. A simple test using grooved pegs clearly demonstrated the poor visual-motor integration in VLBW and LBW children. A marked difference between the NBW and the two LBW groups was seen in a qualitative evaluation of motor performance. Only 5% of VLBW children scored at or above the median for the NBW in all of three fields: general cognitive, motor performance, and pegboard, as opposed to 11% of LBW and 18% of NBW children. Neither sex, age corrected for prematurity, nor psycho-social background factors explained the differences between the groups.(ABSTRACT TRUNCATED AT 250 WORDS)