Little is known about the aetiology of thymoma. This study aims to identify medical risk factors for thymoma as a systematic approach to new hypotheses on the aetiology of this disease. A European multi-centre case–control study was conducted from 1995 to 1997, including incident cases aged 35–69 years with thymoma. Altogether, we accepted 85 cases and 3350 controls, of which we interviewed 77 cases and 2071 population controls about constitutional factors, medical examinations, and former diseases. Odds ratios (ORs) with 95% confidence intervals (CI) were calculated. Medical examinations with X-ray or radiotherapy performed >20 times at least one year before the thymoma diagnosis indicated a possible risk factor for thymoma (OR 1.58, 95% CI 0.93–2.69). Having the first radiotherapy treatment at least one year before the thymoma diagnosis yielded an OR for thymoma of 2.39; 95% CI (0.96–5.99), and if it was at least five years before, the OR for thymoma was 2.81; 95% CI (1.03–7.72). Having a red/auburn hair colour was associated with thymoma, (OR 3.6, 95% CI 1.4–9.5) whereas having pigmented skin was slightly associated with thymoma (OR 1.8, 95% CI 0.8–3.8). Over twenty instances of X-ray examinations or radiotherapy were identified as potential risk factors for thymoma, along with certain constitutional factors. The observed correlations between benign tumours and thymoma could stem from an inherent predisposition to tumour development or result from detection bias. Given that this is the initial analytical study examining medical risk factors for thymoma, all of the results should be approached with caution, acknowledging the possibility that some findings might be incidental. © 2024 by the authors.