The "small baby syndrome hypothesis" suggests that an inverse linear relation exists between birth weight and risk of type 2 diabetes. The authors conducted a meta-analysis to examine this association. They included studies that reported odds ratios and 95% confidence intervals (or data with which to calculate them) for the association of type 2 diabetes with birth weight. Fourteen studies involving a total of 132,180 persons were identified. Low birth weight (<2,500 g), as compared with a birth weight of >/=2,500 g, was associated with increased risk of type 2 diabetes (odds ratio (OR) = 1.32, 95% confidence interval (CI): 1.06, 1.64). High birth weight (>4,000 g), as compared with a birth weight of </=4,000 g, was associated with increased risk to the same extent (OR = 1.27, 95% CI: 1.01, 1.59). Pooled estimates increased further when normal birth weight (2,500-4,000 g) was used as the reference category (low birth weight: OR = 1.47, 95% CI: 1.26, 1.72; high birth weight: OR = 1.36, 95% CI: 1.07, 1.73). Meta-regression and categorical analyses showed a U-shaped relation between birth weight and diabetes risk. These findings indicate that there exists a relation between birth weight and later-life risk of type 2 diabetes which is not linearly inverse but U-shaped.