The effect of support, from dependable non-household relationships, on breast cancer progression was studied using a prospective, longitudinal design.Dependable social support was assessed in women with invasive breast carcinoma Stages II and III within 18 months after diagnosis. Disease outcome was monitored for 8 to 9 years. Cox regression analyses, including the Nottingham Prognostic Index (NPI) score of disease severity, tested the association between the number of dependable support persons and time to death.Ninety participants listed between 1 and 16 dependable nonhousehold support persons (mean=6, S.D.=4). There were 21 recurrences and 16 deaths from breast cancer at the end of the study. The number of dependable supports predicted decreased mortality [RR=0.41 (0.21–0.80), P=.01] after controlling for NPI. NPI predicted increased mortality [RR=1.6 (1.0, 2.4), P=.05].For patients with Stages II and III breast cancer, the number of dependable, nonhousehold relationships predicts decreased mortality, after accounting for disease severity.