Our objective was to improve the currently imperfect classifications of idiopathic inflammatory myopathies (IIM). In clinical practice, overlap features are common in IIM. This provided a rationale for positioning overlap clinical features at the core of a new classification system. We conducted a longitudinal study of 100 consecutive adult French Canadian patients with IIM. Clinical and laboratory data were obtained by retrospective chart review. Sera were analyzed for autoantibodies (aAbs) by protein A-assisted immunoprecipitation and double immunodiffusion. Overlap aAbs encompassed aAbs to synthetases, systemic sclerosis-associated aAbs, anti-signal recognition particle (SRP) and anti-nucleoporins. Patients were classified both at IIM diagnosis, based on data at presentation, and at the end of follow-up, based on cumulative findings. Three classifications were used: 1) the Bohan and Peter original classification, 2) a new version of that classification as modified by us, and 3) a novel clinicoserologic classification. As investigators were blinded to aAb results, the modified classification is strictly a clinical classification. Its core concept is the attribution of diagnostic significance to the presence of overlap features, that is, their presence resulted in a diagnosis of overlap myositis (OM). This approach allowed direct comparison with the original Bohan and Peter classification. By integrating aAb results to the modified classification, we also defined the clinicoserologic classification, which allowed to examine the added value of aAbs to diagnostic, therapeutic and prognostic stratification. Whereas polymyositis (PM) was the most common IIM according to the original classification, accounting for 45% of the cohort at diagnosis, its frequency fell to 14% with the modified classification. Conversely, while the frequency of myositis associated with connective tissue disease was 24% according to the original classification, the frequency of OM was 60% when using the modified classification. At last follow-up, the frequency of PM fell further to only 9%, while the frequency of OM rose to 67%. Systemic sclerosis was the most common connective tissue disease associated with IIM, accounting for 42.6% of OM patients and 29% of the cohort. The frequencies of overlap aAbs in the cohort and in OM patients were 48% and 70.5% (n =48/68), respectively. The presence of overlap aAbs at IIM diagnosis identified additional OM patients unrecognized by the modified classification. The sensitivity of the modified classification for OM at diagnosis was 87%, suggesting that clinicians may rely on the modified classification for identification of most OM patients, while awaiting results of aAb assays. The new classifications predicted the response to prednisone and IIM course. Using stringent definitions, IIM was classified as responsive or refractory after an adequate initial corticosteroid therapy, and the disease course as monophasic or chronic after a single adequate trial of prednisone. PM was always chronic and was associated with the highest rate (50%) of refractoriness to initial corticosteroid treatment. Dermatomyositis was almost always chronic (92% rate); however, its responsiveness to initial corticosteroid treatment was high (87%). OM was almost always responsive to corticosteroids (89%-100% rates). When OM patients were divided according to aAb subsets, anti-synthetase, SRP, or nucleoporin aAbs were markers for chronic myositis, whereas aAbs to U1RNP, Pm-Scl, or Ku were markers for monophasic myositis. We conclude that the original Bohan and Peter classification should be abandoned as it leads to misclassification of patients. Much of IIM is composed of OM. The proposed modified and clinicoserologic classifications have diagnostic, prognostic, and therapeutic value.