OBJECTIVE Pancreatic β-cell mass increases rapidly during gestation and early infancy. Infants who are small for gestational age, which is a marker for poor intrauterine nutrition, have reduced β-cell mass. We examined whether weight and length in early infancy, and in inference β-cell mass, is related to age at onset of insulindependent diabetes mellitus (IDDM). RESEARCH DESIGN AND METHODS Data from infant records of 232 patients with IDDM, including birth weight, birth length, gestational age, weight at 6 months of age, and feeding history during the first 6 months of life, were analyzed. Maternal recall was not used for data collection. RESULTS Low-birth-weight infants (P < 0.0001). Infants small for gestational age also had earlier onset than those with birth weight above the 10th percentile after correction for gestational age (6.2 [3.6–10.5] vs. 9.2 [5.4–11.8] years; P < 0.0001). Infants with corrected birth weight: length ratio below the 10th percentile had earlier onset, as did infants with corrected 6-months weight below the 10th percentile (4.9 [2.8–6.0] years vs. 8.8 [5.2–11.8] years; P < 0.0001). Infants who were exclusively breast-fed for 6 months showed a slightly later onset of diabetes than those who were bottle- or mixed-fed, independent of weight (9.4 [5.0–11.3] years vs. 8.3 [4.2–11.7] years; P < 0.0001). CONCLUSIONS Weight and growth parameters in utero and early infancy may influence the age at onset of IDDM. β-cell mass is likely to be a significant factor.