We measured annual prevalence of microbiologically defined nontuberculous mycobacterial lung disease in Ontario, Canada.Mycobacterium aviumprevalence was 13 cases/100,000 persons in 2020, a 2.5-fold increase from 2010, indicating a large increase in trueM. aviumlung disease. During the same period,M. xenopidecreased nearly 50%, to 0.84 cases/100,000 persons.