Objective This article aims to describe the time trend in number of dual diagnosis patients treated in the psychiatric system in Denmark from 2000 to 2017. Method We calculated the share of patients with dual diagnosis, number of dual diagnosis contacts, number of unique individuals with dual diagnosis as well as number of new patients with dual diagnosis among patients in psychiatric treatment, i.e. among inpatients, outpatients and patients in emergency departments. In order to calculate this, we merged data from the National Patient Register (NPR), the National Registry of Alcohol Treatment, the National treatment registry for substance use, the National Prescription Registry and the Danish National Health Service register in the period from 2000 to 2017. Results We found an overall increase in patients with dual diagnosis in psychiatric treatment in Denmark from 2000 to 2017. We further detected an increase in the age and sex-standardized number of patients with dual diagnosis in treatment over time, however most markedly for outpatients. Further, inclusion of data from other sources of data than the NPR dramatically increased the number of patients that could be identified as dual diagnosis patients. Using these data, almost half of all male inpatients could be identified as dual diagnosis while the share was more than 40% for patients with schizophrenia, schizotypal and delusional disorders (F2) and patients with personality disorders (F6). Conclusions The increase of individual diagnosis patients necessitates action at different levels. This includes improvement of preventive measures as well as improvement of treatment for this underserved group.