Introduction and objectives Poorly kept or illegible medical records may be detrimental to patient care while also leaving doctors vulnerable to litigation. Inconsistent data concerning legibility of physicians' handwritten medical notes has been confounded by factors including the clinical experience and the healthcare role of the reader. The present study investigated the rate of illegibility in medical notes recorded by doctors in Irish hospitals and examined whether this rate varied according to hospital department, the reader's clinical experience or healthcare occupation. Design The legibility of twenty-five anonymized random samples of case notes from four university hospitals was analysed by a range of medical, nursing ancillary healthcare staff and legal professionals using a standardized point scale and questionnaire. Results The mean rate of illegibility of samples as scored by 239 observers was 22.2%. The reported rate was lower for surgical versus medical departments (Mann-Whitney U 1/4 233, z 1/4 3.61, P<0.001), varied depending on healthcare occupation (Kruskal-Wallis X2 1/4 62.95, P<0.001), and improved with increased clinical experience (Mann-Whitney U 1/4 370.5, z 1/4 2.03, P<0.05) with legal professionals scoring an illegibility rate of 31%. Conclusions Estimations of legibility of medical records are influenced by observer attributes and possibly by the discipline from which such case notes originate. Our findings suggest a need for continued vigilance in this area.