The rising incidence rates of skin cancer (SC) lead to an enormous burden on healthcare systems. General practitioners (GPs) might play an important part in SC care, but research has shown poor clinical recognition of SC, leading to a high rate of potentially unnecessary referrals. The aim of this study was to evaluate if a dermato-oncological training programme (DOTP) for GPs improved their diagnostic skills and quality of referrals. Out of 194 GPs in the Nijmegen area, 83 (42·8%) followed a DOTP on SC. Referrals from both a trained cohort (TC) and two cohorts of untrained GPs [untrained present cohort (UPC) and untrained historical cohort (UHC)] were included. Data on diagnostic skills, quality of referrals and the number of potentially unnecessary referrals were evaluated. A total number of 1662 referrals were analysed. The referral diagnosis was correct more often in the TC (70·3%) compared with the UPC (56·2%; P < 0·001) and the UHC (51·6%; P < 0·001). Furthermore, the TC also provided a better lesion description, mentioned a diagnosis more often in their referral letters and more often performed diagnostics before referral. In addition, fewer potentially unnecessary referrals were identified in the TC compared with the UPC (62·7% vs. 73·7%; P < 0·001) and the UHC (75·2%; P < 0·001). GPs who followed a DOTP had better diagnostic skills and quality of referrals than untrained GPs, leading to fewer potentially unnecessary referrals. This might enhance a more efficient use of the limited capacity in secondary dermatological care and consequently lead to lower healthcare costs. © 2020 Radboudumc. British Journal of Dermatology published by John Wiley & Sons Ltd on behalf of British Association of Dermatologists.