Context The Common Terminology Criteria for Adverse Events (CTCAE) is the basis for standardized clinician-based grading and reporting of adverse events in cancer clinical trials. The U.S. National Cancer Institute has developed the Patient-Reported Outcomes version of the CTCAE (PRO-CTCAE) to incorporate patient self-reporting of symptomatic adverse events. Objectives The aim of the study was to translate and linguistically validate a Danish language version of PRO-CTCAE. Methods The U.S. English language PRO-CTCAE was translated into Danish using forward and backward procedures with reconciliation. The linguistic validity of the PRO-CTCAE Danish was examined in two successive rounds of semistructured cognitive interviews in a sample of 56 patients equally distributed by gender and cancer type (prostate, head and neck, lung, breast, gynecological, gastrointestinal, and hematological cancer), and who were currently undergoing cancer treatment. Results In the first round of linguistic validation (n = 42), the phrasing of five symptomatic toxicities was adjusted, and the refined phrasing was retested in a second round of interviews (n = 14). Agreement about phrasing that was both culturally acceptable and semantically comprehensible was achieved in the second round. Statements from participants describing the meaning of the PRO-CTCAE symptomatic toxicities support conceptual equivalence to the U.S. English language version. Conclusion Availability of the NCI PRO-CTCAE in languages beyond English will support international congruence in self-reporting of side effects of cancer treatment. A rigorous methodology was used to develop the Danish language version of PRO-CTCAE. Results provide preliminary support for the use of PRO-CTCAE in cancer clinical trials that include Danish speakers.