Purpose Administrative discharge registers could be a valuable and easily accessible single-sources for research data on periprosthetic hip joint infection. The aim of this study was to estimate the positive predictive value of the International Classification of Disease 10th revision (ICD-10) periprosthetic hip joint infection diagnosis code in the Danish National Patient Register. Methods Patients were identified with an ICD-10 discharge diagnosis code of T84.5 (“Infection and inflammatory reaction due to internal joint prosthesis”) in association with hip-joint associated surgical procedure codes in The Danish National Patient Register. Medical records of the identified patients (n = 283) were verified for the existence of a periprosthetic hip joint infection. Positive predictive values with 95% confidence intervals (95% CI) were calculated. Results A T84.5 diagnosis code irrespective of the associated surgical procedure code had a positive predictive value of 85% (95% CI: 80-89). Stratified to T84.5 in combination with an infection-specific surgical procedure code the positive predictive value increased to 86% (95% CI: 80-91), and in combination with a noninfection-specific surgical procedure code decreased to 82% (95% CI: 72-89). Conclusions Misclassification must be expected and taken into consideration when using administrative discharge registers for epidemiological research on periprosthetic hip joint infection. We believe that the periprosthetic hip joint infection diagnosis code can be of use in future single-source register based studies, but preferably should be used in combination with alternate data sources to ensure higher validity.