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.