Lack of familial support, particularly from fathers or partners, has been identified by the U.S. Surgeon General as a barrier to breastfeeding. Although partners have been identified as an important source of breastfeeding support, research on U.S. men's knowledge about and attitudes towards breastfeeding is limited. An internet panel survey of 502 U.S. males aged 21-44 years was conducted. It included the 17-item Iowa Infant Feeding Attitude Scale (IIFAS), a series of questions assessing prior exposure to breastfeeding and demographic questions. Frequencies, proportions and means were calculated and analysis of variance used to test differences in the mean IIFAS scale score across demographic groups. A multiple linear regression model was used to identify predictors of IIFAS score. The sample was largely white, non-Hispanic (65.7%), college educated (44.4%) and married (47%). The mean IIFAS score was 57 (SD = 8.13; range 25-84). In the adjusted regression model, being white, non-Hispanic, having a college education, having siblings who were breastfed and observing ten or more different women breastfeeding were significantly associated with higher IIFAS scores. In a sample of U.S. men of reproductive age, breastfeeding knowledge and attitudes as measured by the IIFAS are neutral. Race, education, and exposure to breastfeeding are important predictors of breastfeeding attitudes in males as measured by the IIFAS.