In conjunction with other health professionals, doctors believe they play an important role in promoting breastfeeding to women. Although many have positive breastfeeding attitudes, significant knowledge deficits often limit their capacity to effectively encourage, support and assist breastfeeding women and their infants. Personal breastfeeding experience (of self or partner) may be the main source of breastfeeding knowledge and skill development and is related to improved knowledge, more positive attitudes and greater confidence. This paper describes the relationship between the cumulative length of personal breastfeeding experience and the breastfeeding knowledge and attitudes of a cohort of Australian general practice (GP) registrars, as well as their confidence and perceived effectiveness assisting breastfeeding women. The Australian Breastfeeding Knowledge and Attitude Questionnaire containing demographic items, a 20-item attitude scale and a 40-item knowledge scale was distributed between February and May 2007 to Australian GP registrars in their final year of training. Participants with more than 52-week cumulative personal (self or partner) breastfeeding experience had the highest mean knowledge score, had more positive attitudes, and were more confident and effective than all other participants. Parents with limited personal experience (<or=26 weeks) had the poorest breastfeeding attitudes and their knowledge base was similar to participants with no personal experience. Confidence and perceived effectiveness when assisting breastfeeding women rose with increasing cumulative breastfeeding experience. Personal breastfeeding experience per se does not guarantee better breastfeeding knowledge or attitudes although increasing length of experience is related to higher knowledge, attitude, confidence and perceived effectiveness scores.