A new self-report instrument, the Assessment Instrument of Problem-focused Coping (APC) developed from qualitative interviews, is described. This instrument provides knowledge of the patients' own competence in coping with activities of daily living (ADL), the patients' own assessment of what they experience as problems, and the extent to which they are satisfied with their ADL. The purpose of the study was to test the reliability of the instrument with regard to intra-rater reliability and internal consistency. The study group comprised 40 patients with muscular weakness and other symptoms relating to the postpolio syndrome. The result showed an acceptable internal consistency (alpha 0.70), which confirms the construct validity of the instrument. The test-retest showed that the stability over a period of time varied from low to high for a total of 28 items. At the same time, it is evident that the instrument does not achieve the aim of being a good evaluation instrument, because the stability over a period of time was unsatisfactory. The test-retest should be repeated with a larger test group in future research projects.