OBJECTIVES: Sickness certification is a common task, which is however insufficiently studied. Our objective was to describe, prospectively, prescription practices in sickness certification by primary care physicians. METHODS: We recorded patients receiving a sickness certificate during a six-week study period. The main outcome measures were: duration of sick-leave according to age, profession, diagnosis, nationality, somatic, or psychiatric comorbidity as well as co-factors related to the familial or professional environment. RESULTS: Out of a total of 6,433 consultations, 602 patients received a sickness certification, and in 56% of these, sick-leave duration was > or = 6 days. Multivariate analysis showed that presence of co-morbidity and co-factors, greater age and musculoskeletal, cardiovascular, psychiatric disease and injury were independently associated with a longer sickness certification duration. CONCLUSIONS: Sickness certification is a complex task which entails not only consideration of the diagnosis but also of other factors such as co-morbidity, as well as familial and professional environment. Physicians should be aware of these elements and of situations, which might lead to a longer sick-leave period.