OBJECTIVES: Treatment of breast cancer with Tamoxifen has been reported debatably associated with endometrial cancer. This retrospective cohort study is aiming to track breast cancer patients to find out if Tamoxifen usage is associated with occurrence to endometrial cancer in a commercially insured claims outpatient setting. METHODS: A cohort of 5037 breast cancer patients diagnosed between July 2000 and June 2001 was established from Solucient outpatient database with prior 6 month defined as cohort clearance time. Based on the data warehouse the cohort had been followed up until June 2005 as observation period. Patients were divided into three groups based on the cumulative duration of Tamoxifen use (A, 0 years; B, 1-2 years; C, >3-4 years). Cox proportional hazard regression model was used to examine the relationships between endometrial cancer and Tamoxifen usage with influencing factors age, region and cormorbidities controlled. X2 and Hazard Ratio (HR) were calculated. RESULTS: Endometrial cancer was found in 39/3,831 (1.02%) breast cancer patients in group A, 6/786(0.76%) in group B, and 4/420(0.95%) in group C. No significant difference was observed among the 3 groups (X2 =0.441, p=0.802). In Cox Proportional Hazard regression model, Tamoxifen was not found to have had significant effect (HR = 0.967, P=0.256) and none of the influencing factors including age, region, diabetes mellitus and hypertension were found to significantly contribute to occurrence of the events (P>0.05). CONCLUSION: The study showed no association between use of Tamoxifen of breast cancer and endometrial cancer occurrence in four years of retrospective cohort observation. The observation period may not be long enough and extended period is needed to confirm the finding.