In a systematic review, we identified 21 separate studies with data on the association between rapid infancy weight gain, up to age 2 y, and subsequent obesity risk. Uniformly all studies reported significant positive associations. We transformed the reported effect sizes to a standard infancy weight gain exposure, and found that further differences in study design accounted for much of the variation in risk. An accompanying paper by Melinda Yeung reminds us that there are benefits of postnatal catch-up growth in certain populations, and suggests that genetic and nutritional factors could moderate the unhealthy translation of rapid infancy weight gain to visceral fat and insulin resistance. Further evidence is needed, and we will need to rigorously test the benefits and risks of any interventions. However, the concept of "healthy" rapid catch-up infancy growth is an attractive prospect. CONCLUSION: Rapid infancy weight gain is consistently associated with increased subsequent obesity risk, but the predictive ability of different weight gain cut-offs needs to be tested.