An antenatal/postnatal intervention involving proactive telephone support and written materials was conducted among primiparas. 400 women, from the Split-Dalmatia County, Croatia, were randomised between November 2013 and December 2016 into three groups: intervention (IG), active control (ACG) and standard care (SCG). Primary outcome was exclusive breastfeeding (EBF) at 3 months. Secondary outcomes included breastfeeding difficulties, attitudes towards infant feeding, breastfeeding self-efficacy and social support. Practice staff were blinded to group allocation. Of 400 women, 45 (11%) were lost to follow-up, and final analyses were conducted on 129 (IG), 103 (ACG) and 123 (SCG) participants. EBF rates at 3 months were significantly higher for the IG (OR 4.6, 95% CI 2.7 to 8.1; EBF 81%) as well as at 6 months (OR 15.7, 95% CI 9.1 to 27.1; EBF 64%) compared to SCG (EBF 47% at 3 months, and 3% at 6 months). Higher rates were also observed for the ACG at 3 months (OR 2.2, 95%CI 1.3 to 3.8, EBF 68%) and 6 months (OR 2.3, 95%CI 1.4 to 3.9, EBF 16%). Participants in the IG had the highest increase in positive attitudes towards infant feeding, in comparison to baseline, and significantly higher breastfeeding self-efficacy. Participants in SCG experienced significantly more breastfeeding difficulties, both at 3 and 6 months, in comparison to AC and IGs. Written breastfeeding materials and proactive telephone support among primiparas are an effective means of increasing breastfeeding rates, decreasing breastfeeding difficulties and improving self-efficacy and attitudes towards infant feeding.