ObjectiveTo elucidate the prevalence of Japanese ADSSL1 myopathy and determine the clinicopathologic features of the disease.MethodsWe searched for ADSSL1 variants in myopathic patients from January 1978 to March 2019 in our repository and assessed the clinicopathologic features of patients with variants.ResultsWe identified 63 patients from 59 families with biallelic variants of ADSSL1. Among the 7 distinct variants identified, c.781G>A and c.919delA accounted for 53.2% and 40.5% of alleles, respectively, suggesting the presence of common founders, while the other 5 were novel. Most of the identified patients displayed more variable muscle symptoms, including symptoms in the proximal and/or distal leg muscles, tongue, masseter, diaphragm, and paraspinal muscles, in adolescence than previously reported patients. Dysphagia with masticatory dysfunction developed in 26 out of 63 patients; hypertrophic cardiomyopathy developed in 12 out of 48 patients; and restrictive ventilatory insufficiency developed in 26 out of 34 patients in later stages. Radiologically, fat infiltration into the periphery of vastus lateralis, gastrocnemius, and soleus muscles was observed in all patients. Pathologically, nemaline bodies in addition to increased lipid droplets and myofibrillar disorganization were commonly observed in all patients, suggesting that the disease may be classified as nemaline myopathy. This finding revealed that ADSSL1 myopathy is the most frequent among all genetically diagnosable nemaline myopathies in our center.ConclusionsADSSL1 myopathy is characterized by more variable manifestations than previously reported. It is the most common among all genetically diagnosable nemaline myopathies in our center, although mildly increased lipid droplets are also constantly observed features.