BACKGROUND: Although numerous cross-sectional studies have shown an association between WHO infant and young child feeding (IYCF) indicators and child anthropometric measures, limited longitudinal evidence exists linking these indicators with subsequent growth. OBJECTIVES: The purpose of this study was to investigate whether meeting WHO IYCF indicators at 6 and 12 mo of age was associated with growth to 18 mo of age and if dietary diversity mediated the relation between household wealth, maternal education, and child growth. METHODS: We used longitudinal data on 811 infants in the CIGNIS (Chilenje Infant Growth, Nutrition, Infection Study), a randomized controlled trial comparing the effect of micronutrient-fortified porridges on infant growth in Lusaka, Zambia. Twenty-four-h diet recalls were conducted at 6 and 12 mo of age, and length and weight measurements at ages 6 and 18 mo were used to produce height-for-age Z-scores (HAZs) and weight-for-height Z-scores (WHZs). Information on household assets was used to generate a household wealth index, and level of maternal education was collected. RESULTS: In fully adjusted analyses, iron-rich food intake at 6 mo and greater household wealth and maternal education were positively associated with HAZ at 18 mo (all P ≤ 0.016). Iron-rich food intake at 6 and 12 mo, achieving a "minimum acceptable diet" at 12 mo, and higher maternal education were associated with greater WHZ at 18 mo (all P ≤ 0.044). Dietary diversity at 6 mo of age was positively associated with both HAZ and WHZ at 18 mo (both P ≤ 0.001) and mediated 13.4% and 25.9% of the total effect of maternal education on HAZ and WHZ, respectively, at 18 mo. CONCLUSIONS: Our findings indicate that IYCF programs should be targeted toward the early period of complementary food introduction and that policies aimed at increasing formal maternal education may benefit child growth through improved feeding practices. This trial was registered at www.controlled-trials.com as ISRCTN37460449.