We examined the effectiveness of a peer counseling breastfeeding support program for low income women in Michigan who participate in WIC. Because there was more demand for services than could be met by the program, many women who requested services were not subsequently contacted by a peer counselor. We used a quasi-experimental methodology that utilized this excess demand for services to estimate the causal effect of the support program on several breastfeeding outcomes. We relied on data derived from administrative and survey-based sources. After providing affirmative evidence that our key assumption is consistent with the data, we estimated that the program caused the breastfeeding initiation to increase by about 27 percentage points and the mean duration of breastfeeding to increase by more than 3 weeks. The support program we evaluated was very effective at increasing breastfeeding among low income women who participate in WIC, a population that nationally breastfeeds at rates well below the national average and below what is recommended by public health professionals. Given the substantial evidence that breastfeeding is beneficial for both the child and mother, the peer counseling breastfeeding support program should be subjected to a cost/benefit analysis and evaluated at other locales.