BACKGROUND: Being born small for gestational age (SGA) is a risk factor for later development of type 2 diabetes. The development of glucose tolerance disorders in adults involves insulin resistance and impaired insulin secretion. OBJECTIVE: To evaluate insulin secretion and insulin sensitivity in a 4-yr old cohort of SGA. METHODS: 85 children were prospectively followed from mid-gestation to 4 years of age. Fetal growth velocity (FGV) was measured using ultrasound measurements. Body composition and hormonal profile were measured at birth, 1 and 4 years. RESULTS: 23 SGA babies had lower birth weight compared to 62 AGA (-1.9±0.3 vs. -0.6±0.8 z-score; p<0.0001) and they were thinner at birth (ponderal index 24.8±1.8 vs. 26.3±3.1 kg/m3; p = 0.01 and fat mass 11±2.6 vs. 12.9±3.1%; p = 0.01). No significant differences in other measured metabolic and hormonal parameters were observed between two groups at birth. SGA infants experienced an early catch-up growth in weight (mean gain of 1.1±0.6 SD) during the first year of life. At 4 years, SGA children remain lighter than AGA, but with weight z-score in the normal range (-0.1±1.3 vs. 0.5±1.3 z-score; p = 0.05). No excess of fat mass was observed (19±4.8 vs. 19.7±4.1%; p = 0.45). 120-min plasma glucose was significantly higher (6.2±1.1 vs. 5.6±0.9 mmol/l; p = 0.006) and insulinogenic index was significantly lower (0.28±0.15 vs. 0.40±2.4; p = 0.02) in the SGA group at 4-yrs of life contrasting with a preserved insulin sensitivity (QUICKI 0.47±0.09 vs. 0.43±0.05; p = 0.06). CONCLUSION: SGA children with compensatory catch-up growth in first year of life show mild disturbances of glucose tolerance associated to a lower insulinogenic index at 4-yrs of age suggesting impairment of ß-cell function.