The objective of this study is to determine the relation between the size of the corpus callosum (CC) and motor performance in a population-based cohort of preterm children. Preterm born children (n = 221) with a gestational age less than or equal to 32 weeks and/or a birth weight below 1500 g were eligible for this study. At the age of 7 or 8 years, frontal, middle, posterior, and total areas (mm2) of the corpus callosum were measured on true midsagittal MRI. Due to anxiety of 10 children and motion artifacts in 7 other children, 204 MRIs could be assessed in the preterm group (mean GA 29.4 weeks, sd 2.0,mean BW 1200 g, sd 323). The preterm group consisted of 15 children with cerebral palsy (CP) and 189 children without CP. Motor function was established by using the Movement Assessment Battery for Children, and the Developmental Test of Visual Motor Integration was obtained. The same examinations were performed in 21 term born children. The mean total cross-sectional CC area was significantly smaller in preterm born infants compared with their term born controls (338 mm2 versus 422 mm2, P < 0.0001). The preterm children with CP had significantly smaller mean CC areas compared with the preterms who did not develop CP (P < 0.0001-P < 0.002). However, the preterms born without CP also had significantly smaller body, posterior, and total CC areas compared with term born controls (P < 0.0001-P < 0.002). Only the difference in frontal area measurements dilrc) -3.3 mm2/score point (95% CI -4.5, -2.1). The association existed in all parts of the CC but increased in the direction of the posterior part: frontal: lrc -0.8 mm2/score point (-1.2, -0.4), middle: lrc -1.1 mm2/score point (-1.7, -0.5) and posterior: lrc -1.4 mm2/score point (-1.8, -0.9). An association between CC area and its subareas and the standard scores of the VMI was also found. A larger CC was strongly related t o better scores onthe VMI test total area CC: lrc 0.05 score/mm2 (95% CI 0.03, 0.07), frontal: lrc 0.12 score/mm2 (0.05,0.19), middle: lrc 0.10 score/mm2 (0.05, 0.15) and posterior: lrc 0.12 score/mm2 (0.06, 0.18). After adjustment for gestational age, birth weight, and total cerebral area, these associations were still significant. There is a strong association between the size of the corpus callosum (total midsagittal cross area as well as frontal, middle, and posterior area) and motor function in preterm children, investigated at school age. A poorer score on the Movement ABC was related to a smaller CC. A larger CC was strongly associated with better VMI standard scores.