The aim of this study was to describe which complaints patients themselves regard as the cause of sickness certification. During one week in April 1986, 1,379 patients in Buskerud county, Norway, filled in a form after receiving an initial certificate of illness or a continuation certificate from a general practitioner. International Classification of Primary Care (ICPC) no. 1-29 was used to classify the patients' symptoms and complaints. More than half of the patients (53%) considered pain in the musculoskeletal system, particularly back pain (22%), as the reason for their sickness certification. Physical work load was assumed to be a contributory cause to the complaints by 66% of the patients certified sick because of back pain, 58% of those certified sick due to cervical spine and shoulder symptoms, and 72% of those with myalgia, fibrositis (ICPC no. L18, L19). 29% of the patients meant that their health problems could have been prevented, and 15% reported that they could have continued work if their jobs had been adjusted because of their condition.