In Bangladesh, mothers rarely breastfeed exclusively. The national programme promoting exclusive breastfeeding has focused mainly on hospitals, but the majority of mothers deliver at home, and it is important to reach them there. This research aimed to assess the impact of community-based peer counsellors on the prevalence of exclusive breastfeeding. Other objectives were to assess the impact on infant morbidity and growth, and the acceptability of the lactational amenorrhoea method (LAM) for contraception. In Dhaka, 40 localities were randomised as intervention and control clusters. One woman in each intervention cluster was trained as a peer counsellor. The intervention comprised a minimum of fifteen counselling visits (two in the third trimester of pregnancy, two within 5 days of delivery, one between 10-14 days and then fortnightly), to help mothers establish and continue exclusive breastfeeding for 5 months. A total of 726 mothers, with mean of 4.5 years of school, from the lower-middle socioeconomic class, were enrolled during 1996, of whom 573 completed 5 months of follow-up. In the intervention group, 69% of mothers fed their infants colostrum as the first food compared to 11% of the controls (p<0.0001), and 70% breastfed exclusively for 5 months versus 6% of the controls (p<0.0001). Despite small differences in morbidity from diarrhoea, cough and fever, there was a substantial impact on weight gain. At the end of 5 months, the mean weight-for-length Z score was +0.1 (SD 0.8) in the intervention group and -0.9 (SD 0.8) in the control group (p<0.0001). The majority of mothers who breastfed exclusively, also accepted and practised the LAM (59% at 5 months). This trial has demonstrated that trained community-based peer counsellors can significantly increase exclusive breastfeeding and appropriate contraceptive practices, with benefits to infant health. Inclusion of peer counsellors in mother and child health programmes is recommended.