We reviewed 21 studies (8 blinded and 13 open) on the treatment of botulinum toxin type A for cervical dystonia, directed to the health aspects used to evaluate the patients' response to treatment (Medline search 1985-1993, English language literature). The prerequisite for comparing the treatment results was that studies had to evaluate similar aspects of disease. The ICIDH model, outlined by the World Health Organization in 1980, orders the different health outcomes into distinct classes of disease consequences. Our aim was to order the health outcomes according to the model and, thus, to study the comparability of treatment outcomes. Three differences could be identified between the objective and the subjective instruments. (a) The aspects measured by the subjective instruments varied substantially; of the 22 different subjective instruments identified in 18 studies, 8 measured impairments, 5 disability, and 9 could not be classified according to the ICIDH model. The objective instruments measured impairments. (b) All objective instruments were multiitem, whereas only 2 of 22 subjective instruments could be identified as multiitem. (c) The subjective instruments were generally poorly documented with regard to the number of items, score range, or grading. We conclude that the treatment outcomes can only be compared on the objective level of assessment and with regard to the patients with painful dystonia. The subjective instruments, particularly those focusing on disease-specific disability, deserve further research. The ICIDH model offers a useful framework for selection, improvement, and development of outcome instruments. Because the model clearly demarcates the different consequences of disease, adoption will enhance the comparability of outcomes in cervical dystonia intervention trials.