Sickness certification implies that a health problem impairs ability to work. However, its assessment is seldom performed by physicians. Our objective was, therefore, to assess the specific influence of functional and environmental limitations on the length of sick-leave prescriptions. We conducted a cross-sectional study in French general teaching practices and recorded 353 initial sick-leave certifications. For each of them, the functional and environmental limitations were collected using the ATCIF questionnaire, derived from the International Classification of Functioning. Data analysis was based on a linear regression multivariate model. Among the functional limitations, 'pain' was the main body function impairment (22% of impairments) and 'mobility' the main activity limitation (48%). An environmental barrier was identified in 39% of sick-listed patients, mainly relating to 'products and technology' (20%), which refers to workplace factors. The prescription was longer in cases of activity limitations relating to 'mobility' and in cases of environmental barriers relating to 'products and technology'. The multivariate model explained 27% of the variability of sick-leave length through diagnosis elements and only 7% through functional and contextual elements. In sick-leave prescription, a functional and contextual approach, in addition to the traditional diagnosis-based approach, could better support patients' shared understanding and follow-up, and accountability towards health authorities.