Objective:Although improving mothers' knowledge about breastfeeding can increase rates and duration of breastfeeding, little is known about the influence of fathers' knowledge. The purpose of this study was to assess the knowledge of mothers and fathers about breastfeeding before and after receiving postpartum advice and its relationship to the frequency of breastfeeding.Methods:A clinical trial was performed with mothers and fathers of normal children born at the Hospital de Clínicas de Porto Alegre, Brazil, between July 1994 and March 1995. The study intervention consisted of postpartum advice supplied by means of a video film discussing basic topics of breastfeeding, an explanatory leaflet, and open discussion after viewing the video. The first 208 couples comprised the control group, the next 197 comprised experimental group 1, and the remaining 196 comprised experimental group 2. Immediately after delivery, mothers and fathers in the three groups answered a test on breastfeeding knowledge; they completed the same test at the end of the first month. All families received home visits at the end of the first, second, fourth, and sixth months, or until breastfeeding ceased. Logistic regression was used to evaluate the association between the mothers' and fathers' knowledge and frequency of breastfeeding.Results:Postpartum advice increased the breastfeeding knowledge of mothers and fathers. The mothers with the highest level of knowledge had a 6.5 times higher chance of exclusively breastfeeding at the end of the third month, and 1.97 times higher chance of continuing breastfeeding to the end of the sixth month compared with other mothers. The fathers' knowledge also significantly influenced breastfeeding rates. The children whose fathers knew more had a 1.76 higher chance of being exclusively breastfed at the end of the first month, and 1.91 higher chance of receiving maternal milk at the end of the third month.Conclusion:A simple, inexpensive strategy can increase the level of breastfeeding knowledge of mothers and fathers and, consequently, have a positive impact on the frequency of breastfeeding.