This analysis employs a national panel study to examine the relationship between marital transition and depression among mothers within the framework of selection and causation processes. The data come from the two-wave, longitudinal National Population Health Survey (NPHS) by Statistics Canada collected in 1994 and again in 1996 focusing on women between 20 and 65 years of age with children living at home (N = 2169). Compared with mothers who remain married, mothers making the transition into single-parenthood had a significantly higher rate of major depression at Time 1, which increased, but not significantly, at Time 2. This suggests that a selection effect may explain the elevated levels of depression among mothers experiencing a marital disruption. Rates of depression among single-parent mothers making the transition into a marital relationship did not decrease significantly between waves nor did the rate differ significantly from stable single-parent mothers at Time 1 or Time 2, suggesting that movement into marriage is not a protective factor.