Purpose Measures of social networks assess the number and nature of a person's social contacts, and strongly predict health outcomes. We explored how social networks change following a stroke and analyzed concurrent and baseline predictors of social networks 6 months poststroke. Method We conducted a prospective longitudinal observational study. Participants were assessed 2 weeks (baseline), 3 months, and 6 months poststroke. Measures comprised the Stroke Social Network Scale (Northcott & Hilari, 2013), Medical Outcomes Study Social Support Survey (Sherbourne & Stewart, 1991), National Institutes of Health Stroke Scale (Brott et al., 1989), Frenchay Aphasia Screening Test (Enderby, Wood, Wade, & Langton Hewer, 1987), Frenchay Activities Index (Wade, Legh-Smith, & Langton Hewer, 1985), and Barthel Index (Mahoney, Wood, & Barthel, 1958). Analyses of variance and standard multiple regression were used to analyze change and identify predictors. Results Eighty-seven participants (37% with aphasia) were recruited; 71 (16% with aphasia) were followed up at 6 months. Social network scores declined poststroke (p = .001). Whereas the Children and Relatives factors remained stable, the Friends factor significantly weakened (p < .001). Concurrent predictors of social network at 6 months were perceived social support, ethnicity, aphasia, and extended activities of daily living (adjusted R2 = .42). There were 2 baseline predictors: premorbid social network and aphasia (adjusted R2 = .60). Conclusions Social networks declined poststroke. Aphasia was the only stroke-related factor measured at the time of the stroke that predicted social network 6 months later.