Hyperinsulinaemia, obesity and dyslipidaemia are independent and collective risk factors for many cancers. The long-term effects of a 23% high fat diet (HFD) in two immunodeficient mouse strains (NOD/SCID and Rag1-/-), and the effect of diet-induced hyperinsulinaemia on human prostate cancer cell line xenograft growth, were investigated. Rag1-/- and NOD/SCID HFD-fed mice demonstrated diet-induced impairments in glucose tolerance at 16 and 23 weeks post weaning. Rag1-/- mice developed significantly higher fasting insulin levels (2.16 ± 1.01ng/ml, P = 0.01) and increased insulin resistance (6.70 ± 1.68 HOMA-IR, P = 0.01) compared to normal chow-fed controls (0.71 ± 0.12ng/ml and 2.91 ± 0.42 HOMA-IR), however, alterations in insulin resistance were not observed in the NOD/SCID strain. Hepatic steatosis and intramyocellular lipid storage was increased in Rag1-/- HFD-fed mice compared to NOD/SCID HFD-fed mice and absent in mice fed normal chow. Hepatic steatosis in HFD-fed mice was more extensive in Rag1-/- compared to NOD/SCID mice. In Rag1-/- HFD-fed mice, both PC3 and LNCaP xenograft tumour growth velocity was greater compared to chow-fed mice. Mice with PC3 xenografts developed significantly greater normalised wet tumour weight (485.16 ± 143.80% compared to 1562.69 ±338.20%, P = 0.032), tumour volume (485.16 ± 143.80% compared to 1562.69 ± 338.20%, P = 0.032) and velocity of proliferating Ki67+ PC3 tumour cells (36.08 ± 2.53% compared to 66.14 ± 8.514, P = 0.032) compared to mice fed normal chow. The percentage of mice surviving to ethical endpoint was significantly decreased in both HFD-fed groups compared to chow-fed mice (P = 0.0078 and P = 0.031). This is the first characterisation of the metabolic effects of long term HFD feeding in two strains of immunodeficient mice suitable for xenograft studies. Rag1-/- mice are an appropriate and novel xenograft model for studying the relationship between cancer and hyperinsulinaemia.