We conducted the current study to determine the prevalence, incidence density, and clinical significance of thyroid disease in a series of 109 patients diagnosed with idiopathic inflammatory myopathy. We analyzed 109 patients diagnosed with idiopathic inflammatory myopathy during a 20-year period (1986-2006). Routine determination of thyroid profile, including hormone levels and antithyroid antibodies (antithyroid peroxidase and antithyroglobulin antibody systematically, and antithyroid-stimulating hormone receptor antibody when appropriate) was carried out at onset and at least once again during the follow-up period. Human leukocyte antigen (HLA) study was performed by polymerase chain reaction sequence-specific oligonucleotide typing. Six patients (5.5%) developed thyroid disease (hypo- or hyperthyroidism), with an incidence density of 8.7 new cases per 1000 person-years; 4 of the 6 cases were of autoimmune origin (3.6%). Thyroid dysfunction, whether autoimmune or not, was clearly related with onset or relapse of disease activity. Neoplastic disease was detected in 3 of 4 (75%) patients with autoimmune thyroid disease, a significantly higher frequency than was observed in the remaining patients in the series (12 of 103, 11.7%; p < 0.05; RR: 22.8; 95% confidence interval, 2.2-236.6). HLA DRB1*04 was found in 4 of the 6 patients with thyroid disease and inflammatory myopathy. Thyroid disorders are clinically relevant in patients with inflammatory myopathies. An association was observed between autoimmune thyroid disease and cancer. Thyroid assessment is strongly recommended in patients with idiopathic inflammatory myopathy and clinical relapse.