Purpose: Despite a uniform regional breast cancer care program, breast cancer survival differs within regions. We therefore examined breast cancer survival in relation to differences in diagnostic activity, tumor characteristics, and treatment in seven Swedish counties within a single health care region. Methods: We conducted a population-based observational study using a clinical breast cancer register in one Swedish health care region. Eligible women (n = 7,656) ages 40 to 69 years diagnosed with primary breast cancer between 1992 and 2002 were followed up until 2003. The 7-year relative survival ratio was used to estimate breast cancer survival. Excess mortality was modeled using Poisson regression to study differences in survival between counties. Results: The 7-year relative survival for breast cancer patients was significantly lower (up to 7% in absolute risk difference) in one county (county A) compared with the others. This difference existed only among women diagnosed before 1998, ages 50 to 59 years, and was strongest among stage II breast cancer patients. Adjustment for amount of diagnostic activity eliminated the survival differences among the counties.