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Introduction/Aims: Although the extent of muscle weakness and organ complications has not been well studied in patients with late‐onset myotonic dystrophy type 1 (DM1), adult‐onset DM1 is associated with severe muscle involvement and possible life‐threatening cardiac and respiratory complications. In this study we aimed to compare the clinical phenotype of adult‐onset vs late‐onset DM1, focusing on the prevalence of cardiac, respiratory, and muscular involvement. Methods: Data were prospectively collected in the Dutch DM1 registry. Results: Two hundred seventy‐five adult‐onset and 66 late‐onset DM1 patients were included. Conduction delay on electrocardiogram was present in 123 of 275 (45%) adult‐onset patients, compared with 24 of 66 (36%) late‐onset patients (P =.218). DM1 subtype did not predict presence of conduction delay (odds ratio [OR] 0.706; confidence interval [CI] 0.405 to 1.230, P =.219). Subtype did predict indication for noninvasive ventilation (NIV) (late onset vs adult onset: OR, 0.254; CI, 0.104 to 0.617; P =.002) and 17% of late‐onset patients required NIV compared with 40% of adult‐onset patients. Muscular Impairment Rating Scale (MIRS) scores were significantly different between subtypes (MIRS 1 to 3 in 66% of adult onset vs 100% of late onset [P <.001]), as were DM1‐activC scores (67 ± 21 in adult onset vs 87 ± 15 in late onset; P <.001). Discussion: Although muscular phenotype was milder in late‐onset compared with adult‐onset DM1, the prevalence of conduction delay was comparable. Moreover, subtype was unable to predict the presence of cardiac conduction delay. Although adult‐onset patients had an increased risk of having an NIV indication, 17% of late‐onset patients required NIV. Despite different muscular phenotypes, screening for multiorgan involvement should be equally thorough in late‐onset as in adult‐onset DM1. [ABSTRACT FROM AUTHOR]