Despite a growing concern for matters related to sick-leave and its economical and human consequences, little Is still known of the practice of sickness certification. To remedy this, a study based on the critical incident technique was designed to explore dilemmas experienced by physicians when issuing sickness certificates. A questionnaire was distributed to 170 general practitioners (GPs), private physicians and psychiatrists in the Swedish county of Ostergotland asking about sickness certification dilemmas, the consequences of the dilemma and how the situation was resolved. Through a semi-qualitative analysis, 2 main types of dilemma were identified. Insurance-associated dilemmas were the most frequent and concerned the grading of work incapacity, the duration of a sick-leave period and the difficulties in interpreting the sickness insurance legislation. The focus on biomedical diagnosis in the sickness certificate was found to complicate the certification routines, since working capacity may be reduced even though a diagnosis has not been confirmed. The primary medical dilemmas consisted of difficulties related to encountered obstacles In the clinical management, e.g. in subjective medical history, diagnosis or patient compliance. It is concluded that the physicians' role in the insurance system makes it necessary to give the medical diagnosis a social interpretation. The development of standards for grading of work incapacity is needed as well as routines for closer cooperation between the different actors in the sickness insurance system.