An international effort is underway to develop a new international growth reference for assessing the growth of young children, especially breastfed infants who appear to falter relative to the currently recommended National Center for Health Statistics/World Health Organization reference. While limited data from high socioeconomic status children from different parts of the world suggest that their growth patterns are similar, there is no comprehensive study of breastfed infants. The WHO Multinational Study of Breastfeeding and Lactational Amenorrhea provides bi-weekly weights and 2-4 weekly length measurements on breastfed babies from selected sites in Australia, Chile, China, Guatemala, India, Nigeria and Sweden. Multi-level modelling was used to analyse between-site differences in the growth of approximately 120 infants per site, after adjustment for maternal stature and infant feeding pattern. All mothers were literate and mean educational levels were well above national averages, but the study was not restricted to infants of high socioeconomic status. Maternal education was significantly associated with infant weight only in India. The growth curves of infants from most sites were strikingly similar, but relative to the Australians (the reference category), the Chinese babies were about 3% shorter at 12 mo of age and the Indians up to 15% lighter. The present results suggest that breastfed babies from reasonably well-off families in different continents show very similar growth patterns. However, it is important that the growth of children from South and East Asian populations be rigorously assessed in the process of developing the new international growth reference. This paper discusses the relative importance of environmental versus genetic influences in the growth of young children and illustrates the complexities involved in the analysis of growth data.