Abstract Purpose Pharmacovigilance studies of cancer treatment frequently monitor infections. Predictive values of algorithms identifying disease depend on prevalence of the disease in the population under study. We therefore estimated the positive predictive value (PPV) of primary inpatient diagnosis of infection among cancer patients in the Danish National Registry of Patients (DNRP). Methods The algorithm to identify infections in the DNPR was based on International Classification of Diseases , 10th revision (ICD-10) codes. A physician blinded to the type of sampled infection reviewed the medical charts and assessed the presence and type of infection. Using the physician global assessment as gold standard, we computed PPVs with and without requiring agreement on infection type. Results We retrieved 266 of 272 medical charts (98%). Presence of infection was confirmed in 261 patients, resulting in an overall PPV of 98% (95% confidence interval, 96%–99%). When requiring agreement on infection type, overall PPV was 77%. For skin infections, pneumonia, and sepsis, PPVs were 79%, 93% and 84%, respectively. For these infections, we additionally calculated PPVs using evidence-based criteria as the gold standard. PPV was similar for pneumonia, but lower for skin infections and sepsis. Conclusions The Danish National Registry of Patients is suitable for monitoring infections requiring hospitalization among cancer patients.