Estrogen receptor (ER) positive breast cancers represent approximately 75% of all breast cancers (BCa). As an ER positive human BCa line, MCF7 cells are an important tool in BCa research. Their use in xenograft studies typically requires the exogenous input of estrogen to facilitate survival and growth of tumours. However, high concentrations of estrogen can result in renal toxicity and bladder stone formation, which ultimately cause death of the animal. We recently performed a small pilot study to assess if low doses of estrogen were able to facilitate tumour establishment and growth whilst reducing estrogen toxicity in the mice. We injected 1 million MCF7 cells in matrigel orthotopically into Balb/nu mice, and separated them into 6 treatment groups as follows: No estrogen (E2) administration, 0.5mg E2 pellet or 0.3mg E2 pellet for the entire duration of the experiment (continuous treatment), 0.5mg E2 pellet or 0.3mg E2 pellet for 1 week following injection of the MCF7 cells (short-term treatment) or continuous E2 (1µM) administration via drinking water. Tumours established in the no E2 treatment, short-term treatment and treatment via drinking water groups, but no further tumour growth was observed. Tumours also established in the continuous treatment groups and dose-dependent tumour growth was observed at both 0.5mg and 0.3mg E2 concentrations with no mortality or obvious signs of renal toxicity. This study shows that whilst tumours derived from MCF7 cells require continuous estrogen stimulus to grow, as little as 0.3mg of E2 can facilitate this growth with no obvious estrogen related toxicity detected in the animals.