Background The Danish National Patient Registry holds data on hematological procedure codes including date and type of treatment from all hematological departments in Denmark. The validity of the hematological procedure codes remains to be clarified before they are used in epidemiological research. Patients and Methods Using the Danish Myelodysplastic Syndromes Database, we identified 897 patients diagnosed with myelodysplastic syndromes or chronic myelomonocytic leukemia treated at five Danish Hospitals between 1 January 2012 and 30 April 2019. From the Danish National Patient Registry, we ascertained information about hematological procedure codes and date of procedure registered on each patient and generated random samples. Using medical record review as the reference standard, we validated procedure codes in the Danish National Patient Registry and calculated positive predictive values (PPVs) with 95% confidence intervals (CIs) for each procedure code. Results A total of 523 medical records (99% of the total sample) were available for review. PPVs for specific procedure codes ranged from 71% to 100%. The overall PPV was 91% (95% CI: 88%-92%), reflecting PPVs of 95% (95% CI: 92%-97%) for low-dose-chemotherapy, 90% (95% CI: 81%-96%) for high-dose chemotherapy, 99% (95% CI: 93%-100%) for allogeneic stem cell transplantation, 75% (95% CI: 62%-85%) for immuno-modulating agents, 80% (95% CI: 74%-85%) for growth factors, and 99% (95% CI: 99%-100%) for bone marrow examination. The accuracy of coding was consistent across geographic regions and year of registration/coding. Conclusions Hematological procedure codes reported to the Danish National Patient Registry had high PPVs and are suitable for epidemiological research.