BACKGROUND This retrospective study was undertaken to define risk factors for reductions in dose intensity of adjuvant chemotherapy in women with early stage breast cancer (ESBC). METHODS A nationwide survey of 190 community oncology practices was conducted between 1998 and 2002 with data collected retrospectively on 3,707 patients treated with adjuvant chemotherapy for ESBC. End points included reductions in dose intensity, delivered relative dose intensity (RDI) <85%, and the incidence of chemotherapy dose delays >/=7 days and dose reduction >/=15%. Demographic and clinical characteristics, incidence of febrile neutropenia (FN), and patterns of use of granulocyte colony-stimulating factor (G-CSF) were also assessed. RESULTS Average RDI for all regimens was 88%, with 30% of patients receiving <85% of standard for their regimen. Seventeen percent of the reduction in average RDI was planned from the start of therapy, and 13% was unplanned. In univariate analysis, significant predictors of reduced RDI were: age >/=65 years (41%, P < 0.001), body surface area (BSA) >2 m(2) (37%, P < 0.001), negative lymph nodes (33%, P < 0.001), FN (36%, P = 0.013), and comorbidities (40%, P = 0.013), particularly renal disease (86%, P = 0.004). Dose reduction was less with prophylactic G-CSF (24%, P < 0.001). In multivariate analysis, significant independent predictors of reduced RDI included: advanced age, greater BSA, comorbidities, anthracycline-based regimens, a 28-day schedule and FN, while primary G-CSF prophylaxis was associated with a significant reduction in risk. CONCLUSIONS A significant proportion of patients with potentially curable ESBC continue to experience planned and unplanned reductions in RDI.