To examine the validity of registration of hydatidiform mole (HM) in the Danish National Patient Registry (NPR), the Danish Cancer Registry (DCR), and the Danish Pathology Registry (DPR).We selected women registered with a first-time HM code in NPR, DCR, and DPR from 1999 to 2009. We found most women registered in DPR. For a random sample of women registered in DPR, the coding was validated by comparing with the pathology report. Completeness and positive predictive value (PPV) of registration with an HM code in NPR and DCR were calculated using DPR as the reference. Details of women registered in NPR or DCR, but not in DPR, were scrutinized.In NPR and DPR, 1,520 women were identified in total; 1,057 (70%) were found in both registries, 65 (4%) only in NPR, and 398 (26%) only in DPR. In DCR and DPR, 1,498 women were identified in total; 1,174 (78%) in both registries, 47 (3%) only in DCR, and 277 (19%) only in DPR. For 149/150 randomly selected women registered with an HM code in DPR (99%), the pathology report was consistent with the diagnosis of HM. Completeness of NPR was 73% (95% CI: 70%-75%) and PPV was 94% (95% CI: 93%-95%). Completeness of DCR was 72% (95% CI: 69%-75%) in 1999-2003 and 90% (95% CI: 87%-92%) in 2004-2009. PPV of DCR was 96% (95% CI: 95%-97%) throughout the period.Validation of registry data is important before using these. For research on the number of HMs in Denmark, DPR is the most valid data source. NPR and DCR appear to be equally valid before 2004. However, for research after 2004, DCR should be preferred rather than NPR.