OBJECTIVE To compare the costs incurred on infant feeding between the mothers who exclusively breastfed their infants and those who introduced supplements up to 6 months of age. METHODS Longitudinal follow up. METHODS Urban slums of south Delhi. METHODS One hundred normal mother infant pairs fulfilling the prelaid criteria were recruited at the time of birth and followed up for determining the feeding practices. The cost of feeding was estimated at prevalent market prices in terms of food supplement and medical treatment of infant and additional nutritional intake of mothers. The differences in costs in exclusively and partially breastfed groups were analyzed at 3, 4, 5 and 6 months of age. RESULTS There was a sharp decline of exclusive breastfeeding from birth to six months. The mean cost of infant feeding was Rs. 204/- per month in partially breastfed as compared to Rs. 106/- in exclusively breastfed at 6 months of age. The increased cost was largely attributable to supplementary food and the cost of feeding bottles. (83% of mothers used bottles). CONCLUSIONS The mean cost of infant feeding is substantially higher in partially breastfed children.