Background: Many breast cancer outreach programs assume that dissemination of information through social networks and provision of social support will promote screening. The authors prospectively examined the relationship between social network characteristics and adherence to screening guidelines. Method: Employed women age 40 years and older completed baseline and 2-year follow-up assessments (N = 1,475) as part of an intervention trial. The authors modeled screening adherence at follow-up as a function of social network characteristics at baseline. Results: Baseline adherence explained most of the variation in adherence at follow-up. For women age 40 to 51 years, having a mammogram at follow-up was predicted by encouragement by family and/or friends and subjective norms at baseline (odds ratio = 2.20 and 1.18, respectively). For women age 52 years and older, the perception that screening was normative was related to adherence at follow-up (odds ratio = 1.46). Conclusions: Previous mammography use is strongly predictive of future screening. Social network characteristics have a modest impact on screening. Outreach efforts should focus on those who have previously underutilized mammography. (Contains 5 tables.)