Strong associations between low birth weight and insulin resistance have been described. However, most of these studies have been retrospective. We aimed to determine whether infants born small for gestational age (SGA: birth weight <5th percentile for gestational age) have decreased insulin sensitivity, compared with appropriate for gestational age (AGA: birth weight >10th percentile) at 1 yr of age. We studied blood lipids, fasting insulin levels, other markers of insulin sensitivity, and insulin secretion during an iv glucose tolerance test in a cohort of 85 SGA and 23 AGA 1-yr-old infants. In addition, SGA infants were stratified according to catch-up growth (CUG) in weight (WCUG) or length (LCUG) during the first year of life. At 1 yr, SGA infants had a clear tendency to higher triglycerides. Fasting insulin was significantly higher in SGA infants with WCUG, compared with those who did not catch up and AGA infants (mean +/- SEM, 32.6 +/- 4.6 vs. 14.9 +/- 2.3 vs. 21.4 +/- 3.3 pM, respectively; P < 0.05). Length increment (in SD score) was the principal determinant of postload insulin secretion (R(2) = 0.1, P < 0.01). We conclude that insulin secretion and sensitivity are closely linked to patterns of rapid WCUG and LCUG during early postnatal life. Fasting insulin sensitivity is more related to WCUG and current body mass index, whereas insulin secretion seems to be directly related to LCUG.