It is uncertain whether multiple micronutrients benefit the mental and psychomotor development of young children in developing countries. We conducted a randomised double-blind controlled trial to evaluate the effect of a richly micronutrient-fortified v. a basal fortified porridge on mental and psychomotor development in Zambian infants. Infants (n 743) were randomised at age 6 months to receive either the richly fortified or the basal fortified infant food and were followed up until 18 months of age. All the infants were evaluated monthly for achievement of a series of developmental milestones. The Bayley scales of infant development II were administered to a subsample of 502 infants at 6, 12 and 18 months. Rich micronutrient fortification had no significant benefit on the following: (a) number of developmental milestones achieved (rate ratio at 12 months = 1·00; 95 % CI 0·96, 1·05; P = 0·81, adjusted for sex, socio-economic status and maternal education, with similar results at 15 and 18 months); (b) ages of walking unsupported (hazard ratio (HR) 1·04; 95 % CI 0·88, 1·24; P = 0·63, adjusted for the above covariates) and of speaking three or four clear words (HR 1·01; 95 % CI 0·84, 1·20; P = 0·94, adjusted for the above covariates); (c) mental development index (MDI) and psychomotor development index (PDI) of the Bayley scales (scores difference adjusted for baseline scores, age at the assessment, sex, socio-economic status, maternal education, language, age and HIV status: MDI 0·3 (95 % CI − 0·5, 1·1), P = 0·43; PDI − 0·1 (95 % CI − 0·9, 0·7), P = 0·78). In conclusion, the results do not support the hypothesis that rich micronutrient fortification improves Zambian infants' mental and motor development.