To determine the adherence to the national guidelines for start of highly active antiretroviral treatment (HAART) in HIV infected patients. We used a Danish nationwide cohort of HIV infected patients to calculate the fraction of patients who in the period 1997-2006 started HAART according to the guidelines from The Danish Society of Infectious Diseases. We used Kaplan-Meier tables to estimate time from fulfilling the criteria for start of HAART to initiation of the treatment. Cox regression and logistic regression was used to identify risk factors for delayed initiation of treatment and chance of being included in clinical trials. The study included 3223 patients, 74% of whom initiated HAART in the study period. Ninety-four% fulfilled the criteria for start of HAART, with minor differences over calendar periods. Ninety-four% initiated a recommended regimen or were included in a clinical trial. Intravenous drug use predicted initiation of a non-recommended regimen and delay in start of HAART, while non-Caucasians were less likely to be included in clinical trials. In a Western world setting, the adherence to national guidelines for start of HAART can be high. We suggest that simplicity of the guidelines, centralization of treatment and involvement of local clinicians in the development of guidelines are of major importance for high adherence to treatment guidelines. © 2011 The Authors. British Journal of Clinical Pharmacology © 2011 The British Pharmacological Society.