Aims: Sickness certificates are to provide information on a disease and its consequences on the patient's functioning. This information has implications for the patient's rights to sickness benefits and return-to-work measures. The objective of this study was to investigate the description of functioning in sickness certificates according to WHO's International Classification of Functioning, Disability, and Health (ICF), and to describe the influence of patients' age, gender, diagnostic group, and affiliation of certifying physician. Method: A content analysis of written statements regarding how the disease limits the patient's functioning with ICF as a framework was performed in 475 sickness certificates, consecutively collected in Ostergotland County, Sweden. Results: Musculoskeletal diseases (MSD) were the largest diagnostic group, followed by mental disorders (MD). Certificates were mainly issued from physicians at hospitals and in primary health care (PHC). ICF was applicable for classifying statements regarding functioning in 311 certificates (65%). The distribution of components was 58% body functions, 26% activity, and 7% participation. The descriptions were primarily restricted to the use of at least one component; namely, body functions. Subgroup analysis showed that descriptions of activity and participation were more common in certificates for MD and MSD, or those issued by PHC physicians. A multiple regression analysis with the activity component as dependent variable confirmed the results by showing that activity was related to both diagnosis and affiliation. Conclusions: In a consecutive sample of sickness certificates, it was shown that information on functioning is scarce. When functioning was described, it was mainly body oriented.