The number of protocol-eligible patients, refusing to participate in a clinical trial is often not mentioned. The aim of this study is to report the number of refusers and to evaluate the reasons for not participating in a clinical study concerning post-operative pain relief and to assess the potential influence on the final study results. Patients refusing to participate in the study were recorded and evaluated for reasons of refusal. The post-operative pain relief techniques applied in this trial are commonly used, but nevertheless the refusal rate was higher than expected. When it was mentioned that an epidural technique was a part of the trial, 16.7% of the total protocol-eligible group refused. The responses of those offered an epidural could be divided into two groups: the adamant pros and cons to this technique. It can be concluded that in order to be able to judge the validity of results and thus for good clinical practice, the number of patients refusing to participate in a clinical trial and their reasons, should be mentioned in all publications.