We examined social comparisons made by women with breast cancer who participated in peer-support interventions. This setting. in which participants were exposed to similar others, allowed for a longitudinal investigation of the frequency, antecedents. and consequences of social comparison. Consistent with the literature, the majority of comparisons were made to worse-off others (downward), and the majority of comparisons were associated with positive affect (positive comparisons). Low self-esteem. low internal locus of control, and high illness uncertainty were associated with making more negative comparisons. In addition, negative comparisons were associated with a decrease in perceived control and an increase in uncertainty over time. whereas positive doanbard comparisons were associated with an increase in self-esteem. Implications for support groups are discussed.