BACKGROUND:The purpose of this study was to investigate the completeness of TNM (Tumor, Node, Metastasis) staging of melanoma in the Danish Cancer Registry (DCR). METHODS:We identified 8762 patients with a first primary diagnosis of melanoma from the DCR between 2004 and 2009. We obtained information on level of comorbidity, defined according to the Charlson Comorbidity Index, through the Danish National Patient Register. We computed the completeness of TNM staging overall and by each stage component. Analyses were stratified by gender, age, year of diagnosis, and level of comorbidity. We designed an algorithm that categorized melanoma stage as localized, regional, distant, or unknown. Owing to knowledge on clinical coding practice, we allowed for categorization of tumors with certain missing stage components. RESULTS:The overall completeness of the TNM staging was 78.4% (95% confidence interval [CI] 77.5-79.3). Completeness varied little by gender and year of diagnosis. However, completeness decreased from 83.5% (95% CI 81.7-85.3) in patients aged 0-39 years to 68.7% (95% CI 65.7-71.6%) in patients 80 years or older, and from 80.3% (95% CI 79.4-81.3) among patients with a low level of comorbidity to 67.4% (95% CI 63.1-71.4) among patients with a high level of comorbidity. Using the algorithm, 87.3% of cases could be assigned to one of the defined stage categories. CONCLUSION:The overall completeness of the TNM registration for melanoma was fairly high but varied with age and level of comorbidity. Thus, data on TNM stage should be used with caution in epidemiological and other research.