Background: The aim of this study was to examine the phonological functioning (reading speed and accuracy) of hospital patients under general anaesthesia administered during colonoscopy. Methods: In this study the 'Łatysz' non-word reading test was used to measure the impact of selected anaesthetics on the phonological aspect of language processing (defined as decoding without referring to the meaning) in a group of 22 anaesthetised patients compared to 23 non-anaesthetised patients from university clinics. Results: Compared to the preoperative performance, a decrease in reading accuracy and reading speed was observed only in the Anaesthesia Group -AG (in the subjects aged ≥ 35 years) 1.5 h after the administration of anaesthetics. Postoperatively, the AG were significantly slower and less accurate than the Control Group -CG -after 1.5 h. After 3 h, the AG had regained their baseline values both in reading accuracy and reading speed. During the last assessment session, the AG pronounced 82% of the words correctly, while the CG pronounced 74% correctly. Moreover, subjects aged ≥ 35 years performed worse than younger subjects in their reading accuracy and speed. Conclusions: The patients who underwent colonoscopy under general anaesthesia manifested impaired phonological functioning shortly after the procedure, both in the speed and accuracy of reading non-words. However, the accuracy problems subsided relatively quickly.