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. The amount of diagnostic activity was also lower in county A during the same time period. After county A, during 1997-1998, began to adhere strictly to the regional breast cancer care program, neither any survival differences nor diagnostic activity differences were observed. INTERPRETATIONS: Markers of diagnostic activity explained survival differences within our region, and the underlying mechanisms may be several. Low diagnostic activity may entail later diagnosis or inadequate characterization of the tumor and thereby missed treatment opportunities. Strengthening of multidisciplinary management of breast cancer can improve survival.