A large proportion of women with lymph node negative breast cancer do not benefit from chemotherapy. Proliferation markers have been shown to recognize patients at high risk for recurrence. The Ki67 protein has recently been included in the St Gallen guidelines. The authors investigated the prognostic importance of cyclin B1 in node negative breast cancer and included a study of reproducibility. In a population-based case-control study, 190 women who died from breast cancer were defined as cases and 190 women alive at the time for the corresponding case's death were defined as controls. Inclusion criteria were tumor size </=50 mm, no lymph node metastases, and no adjuvant chemotherapy. Tumor tissue was immunostained for cyclin B1. Two investigators (EN-M and AK) evaluated the staining independently by counting approximately 100, 200, 500, and 1000 cells. Cyclin B1 was statistically significantly associated to breast cancer death, in both uni- and multivariate analyses (adjusted for tumor size, age, and endocrine therapy), with odds ratios 2-3 for both investigators. The agreement between the two investigators was good to very good, regardless of the number of counted cells (kappa values between 0.74 and 0.82). Cyclin B1 is a prognostic factor for breast cancer death in a population-based node negative patient cohort which can identify high-risk patients with a good to very good reproducibility.