OBJECTIVE: To quantify the risk of developing post-molar gestational trophoblastic neoplasia (pGTN) beyond the first normal human chorionic gonadotrophin (hCG) in women who have had a complete (CHM) or partial molar pregnancy (PHM) and to re-evaluate the current UK Hydatidiform mole hCG surveillance guidelines. METHODS: The Charing Cross Hospital Trophoblast Disease Centre database was screened to identify all registered cases of hydatidiform mole (HM) between 1980 and 2009. RESULTS: We identified 20,144 cases of HM, comprising 8400 CHM, 9586 PHM, and 2158 cases of unclassified hydatidiform mole (UHM). Twenty-nine cases (20 CHM, 3 PHM and 6 UHM) developed pGTN after the first normal hCG. For CHM the risk of pGTN at the point of hCG normalisation was 1 in 406, and fell rapidly in the first six months of monitoring. For PHM the risk of pGTN at the point of hCG normalisation was 1 in 3195. Women with CHM where hCG normalisation occurred beyond 56days after uterine evacuation of molar tissue were found to have a 3.8-fold higher risk of pGTN. CONCLUSIONS: Our results show that pGTN can occur after hCG normalisation following PHM but the risk is extremely low. Women with CHM have a comparatively higher risk of pGTN after hCG normalisation. Those with CHM where hCG normalises within 56days have a lower risk of pGTN. We have revised the current UK hCG surveillance protocol for PHM to a single additional confirmatory normal urine hCG measurement one month after first normalisation. The protocol for CHM remains unchanged.