Prostate cancer (PCa) is one of the most common types of cancer in men. The aetiology of the disease is not well established, but it has been related to one of the main pathways of regulation of prostate proliferation, mediated by androgens. The androgen receptor (AR) gene encodes the androgen-receptor protein, which functions as a transactivation factor for steroid hormones. It has been proposed that the AR gene transcription levels are mediated by short tandem repeats corresponding to the CAG sequence. However, there are conflicting results in this relationship. We evaluated the expression levels of the AR gene and identified the number of CAG repeats (CAGn) in the Mexican population, establishing the relationship between expression levels and increase in the number of CAG repeats. We evaluated the expression levels of AR in tissue samples of PCa and benign prostate disease, such as benign prostatic hyperplasia, or prostatitis, to determine the difference in their expression levels. Our results showed a statistically insignificant underexpression of 0.64-fold decrease in AR levels of PCa patients compared to benign prostate disease patients (P = 0.623) and suggest that the number of CAGn was correlated with the relative expression of the AR gene (P = 0.009) and this correlation was positive, moderate, and proportional (ρ = 0.467) and no correlation was found between CAGn with other clinical features.