The lack of consensus regarding outcome measures and trial design issues in the idiopathic inflammatory myopathies (IIM) is inhibiting the conduct and interpretation of clinical trials. To begin to address these problems, a multispecialty group of over 70 adult and pediatric neurologists, rheumatologists, rehabilitation medicine physicians, statisticians, and patient support group leaders, called the International Myositis Outcome Assessment Collaborative Study Group (IMACS), is engaged in developing consensus on the assessment of disease activity and damage for myositis clinical trials. As part of this ongoing international effort, members of this group met in November 2001 at a work-shop entitled "Defining Clinical Improvement in Adult and Juvenile Myositis." A goal of the work-shop was to review current data on the validity and responsiveness of the recently published proposed preliminary core set measures for disease outcome assessment in clinical trials for myositis and to define the degree of change in each core set measure that is clinically meaningful. Despite differences in the clinical presentations, natural history and responses to therapy between adult onset and juvenile onset myositis, expert specialists in these diseases came to a consensus that the amount of improvement that is clinically meaningful in each core measure is the same for adult and juvenile myositis. For the domains of muscle strength and physical function, a minimum of 15% improvement is clinically significant, whereas for the physician and patient global assessments, as well as the extramuscular assessment, a minimum of 20% improvement is considered clinically meaningful, and for serum levels of muscle associated enzymes, at least 30% improvement is needed to be clinically important. This workshop is the first of several planned to develop multidisciplinary, international consensus on the conduct and reporting of IIM clinical trials.