There is conflicting evidence in the published literature regarding the effects of clomiphene citrate (CC) on endometrial development and its impact on conception. CC is an anti-oestrogen that is commonly used in assisted reproduction to induce ovulation for a range of conditions. The setting was the Reproductive Medicine department at the Royal Blackburn Hospital (RBH). This retrospective cohort study set out to assess the relationship between endometrial thickness (EMT) as measured by trans-vaginal ultrasound scan and pregnancy rates in 81 sub-fertile women treated with CC and to review some of the current literature. Using analysis of logistic regression, the results showed a 14% increase in the odds of conception for each millimetre increase in EMT when the confounding variables of age, basal follicle-stimulating hormone level and body mass index were corrected for. There is good evidence of a positive relationship between greater EMT (within normal limits) and the probability of success. EMT on its own cannot thus far be used to estimate the probability of conception, rather it seems that changes on a microscopic level give better clues to endometrial quality. Taking into account the limitations of patient selection and some missing data, this study should be viewed as a pilot for future investigation.