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Tamoxifen is a non-steroidal, anti-estrogenic and selective estrogen receptor modulator commonly used as adjuvant chemotherapy in humans with breast cancer and occasionally in dogs following mastectomy. This report presents a case of cystic endometrial hyperplasia-pyometra in a dog following oral administration of the Tamoxifen. A 4-year-old intact nulliparous Boerboel bitch presented at Veterinary Teaching Hospital, Federal University of Agriculture, Abeokuta. It was diagnosed with mammary carcinoma of the right cranial lumbar mammary gland. Mastectomy was performed followed by oral administration of Tamoxifen (10mg daily) for six weeks as adjunct chemotherapy. Eight weeks after, the owner reported that the bitch was lethargic, anorexic and had a purulent vaginal discharge. Abdominal ultrasound revealed cystic endometrial hyperplasia. Results of complete blood counts showed neutrophilic (absolute neutrophil count = 19.5×10³ /L) leukocytosis (total leukocyte count = 24.9X10³ /L), while bacterial culture yielded moderate growth of Staphylococcus aureus, which was sensitive to Ciprofloxacin, Ofloxacin, Sparfloxacin and Gentamycin. The bitch was treated with 400µg of Misoprostol and 500mg of Ciprofloxacin twice daily for two weeks. Ovariohysterectomy (OVH) was performed using a ventral midline approach when no significant improvement was observed from medical treatment. The dog improved significantly after ovariohysterectomy and was discharged one week after OVH. It was concluded that, although tamoxifen is routinely used as adjuvant chemotherapy following mastectomy, more research is required to evaluate its safety in intact bitches. [ABSTRACT FROM AUTHOR]