Compliance is a well recognised but still unresolved health problem; improvement of compliance to treatment would increase cost-effectiveness. One of the current methods used to measure treatment compliance in a clinical drug trial is through the use of a patient diary. In order to interpret data in these diaries it is important to also assess how compliant patients are in completing diaries. Patient compliance of standard diary completion was measured in 69 patients with perennial rhinitis, who were randomised into a double blind, placebo controlled trial with a new corticosteroid drug. During 3 months the patients were instructed to complete a diary twice a day for the following parameters: rhinitis signs and symptoms, dosage times, concomitant medication, use of rescue medication and comments. Diaries were reviewed by the physician at scheduled visits. Twenty patients (30%) completed their diaries for all items perfectly, while 62 patients (94%) completed more than 95% of all items. The compliance of diary completion in a well controlled trial is high. Overall completion of the diary was not influenced by age, gender, race, use of concomitant medication or treatment failure. Significant correlations were found for study duration and physician. This study suggests that completion of a daily diary is positively correlated with patient compliance in medication intake. Physicians could consider using diaries to try to improve compliance. More explicit investigations are needed.