Background: Idiopathic inflammatory myopathies (IIMs) are rare autoimmune diseases with limited epidemiological data from Latin America. Objective: To characterize IIMs through incident rate patterns and clinical features in a major Chilean referral center over a 10-year period. Methods: Historical cohort study (2012-2021) reviewing clinical records from rheumatology outpatient clinic of patients with IIM diagnosis. Incident rates were calculated as IIM cases per specialty consultations. Clinical characteristics, antibody profiles, and treatment outcomes were analyzed. Both consultation-based and population-based estimates for incidence and prevalence were determined. Results: Among 3,594,047 specialty consultations, 100 IIM cases were identified (2.78 cases per 100,000 consultations; 95% confidence interval, 2.27-3.39). Mean annual incidence was 0.58 cases per 100,000 adults (95% confidence interval, 0.47-0.69), with 2021 prevalence ranging from 5.07 to 8.57 per 100,000 adults, depending on the denominator population. Dermatomyositis was the most frequent subtype (71%). Conclusions: This first consultation-based analysis of IIMs in Chile provides baseline data for health care resource utilization. The methodology offers a practical approach for rare disease epidemiology in similar health care settings, whereas the findings align with international reports.