Although phone-based breastfeeding support has been shown to be effective in improving exclusive breastfeeding rates and duration in developed countries, there is paucity of data to demonstrate similar effect in our country Nigeria. The present study was carried out to evaluate the effect of breastfeeding support offered via mobile phone on the duration of exclusive breastfeeding among mothers who delivered in the University of Port Harcourt Teaching Hospital, and to determine the impact of this breastfeeding support on their infants’ weight and length at six months postpartum. The study was designed as a randomized controlled trial with two arms consisting of 75 mothers in each group, i.e. the intervention and control groups. Group assignment was by simple randomization. Women in the intervention group received phone calls on day 7, day 14 and then monthly till six months postpartum with encouragement to breastfeed exclusively and adequately. They were counselled when they had difficulty with breastfeeding. The control group received the standard care. Data on infant feeding were collected by phone from mothers in the two groups. Measurement of weights and lengths of babies in each group were obtained at birth and at six months. At six months, data were collected from 129 mothers consisting of 67 mothers in the intervention and 62 mothers in the control group.