Four hundred and twenty-three patients with probable or definite JDM or juvenile PM (JPM) [18] were enrolled into the NIH Clinical Center or Food and Drug Administration's investigational review board-approved natural history protocols from September 1994 until July 2008; subjects' written consent/assent was obtained according to the Declaration of Helsinki. The study was approved by the NIDDK/NIAMS Institutional Review Board. Enrolled patients provided a blood sample for autoantibody testing and the treating physician completed a questionnaire that included clinical, demographic and laboratory data. For 285 of these patients, questions about factors temporally associated with illness onset were also completed, which is the basis of the present study. Informed consent/parent assent was consistent with the Declaration of Helsinki. Phenotypes were defined by age of illness onset, clinical features, disease course, race or autoantibodies. Disease course was classified as monocyclic if the patient achieved remission without evidence of active disease, based on clinical examination and laboratory testing, within 2 years of diagnosis; as polycyclic if the patient had recurrence of active disease after a definite remission; as chronic continuous if disease activity persisted for >2 years; and as undefined if follow-up was <2 years from the time of diagnosis [8] . Clinical, demographic and autoantibody characteristics of the study population are described in Table 1 . Only the autoantibody phenotypes defined as anti-aminoacyl-tRNA synthetase, anti-signal recognition particle (anti-SRP), anti-Mi2, anti-p155, anti-MJ, anti-U1 RNP and autoantibody negative were included in the analyses of environmental factors.