The reasons for social inequality in breast cancer survival are far from established. Our study aims to study the importance of a range of socioeconomic factors and comorbid disorders on survival after breast cancer surgery in Denmark where the health care system is tax-funded and uniform. All 25,897 Danish women who underwent protocol-based treatment for breast cancer in 1983-1999 were identified in a clinical database and information on socioeconomic variables and both somatic and psychiatric comorbid disorders was obtained from population-based registries. We used Cox proportional hazards models to estimate the association between socioeconomic position and overall survival and further to analyse breast cancer specific deaths in a competing risk set-up regarding all other causes of death as competing risks. The adjusted hazard ratio (HR) for death was reduced in women with higher education (HR, 0.91; 95% confidence interval (CI), 0.85-0.98), with higher income (HR, 0.93; 95% CI, 0.87-0.98) and with larger dwellings (HR, 0.90; 95% CI, 0.85-0.96 for women living in houses larger than 150 m(2)). Presence of comorbid disorders increased the HR. An interaction between income and comorbid disorders resulting in a 15% lower survival 10 year after primary surgery in poor women with low-risk breast cancer having comorbid conditions ( approximately 65%) compared to rich women with similar breast cancer prognosis and comorbid conditions ( approximately 80%) suggests that part of the explanation for the social inequality in survival after breast cancer surgery in Denmark lies in the access to and/or compliance with management of comorbid conditions in poorer women.