Dilated cardiomyopathy (DCM) is characterized by dilation of left ventricular cavity with systolic dysfunction. Clinical symptom of DCM is heart failure, often associated with cardiac sudden death. About 20–35% of DCM patients have apparent family histories and it has been revealed that mutations in genes for sarcomere proteins cause DCM. However, the disease‐causing mutations can be found only in about 17% of Japanese patients with familial DCM. Bcl‐2‐associated athanogene 3 (BAG3) is a co‐chaperone protein with antiapoptotic function, which localizes at Z‐disc in the striated muscles. Recently, BAG3 gene mutations in DCM patients were reported, but the functional abnormalities caused by the mutations are not fully unraveled. In this study, we analyzed 72 Japanese familial DCM patients for mutations in BAG3 and found two mutations, p.Arg218Trp and p.Leu462Pro, in two cases of adult‐onset DCM without skeletal myopathy, which were absent from 400 control subjects. Functional studies at the cellular level revealed that the DCM‐associated BAG3 mutations impaired the Z‐disc assembly and increased the sensitivities to stress‐induced apoptosis. These observations suggested that BAG3 mutations present in 2.8% of Japanese familial DCM patients caused DCM possibly by interfering with Z‐disc assembly and inducing apoptotic cell death under the metabolic stress. 32:1481–1491, 2011. ©2011 Wiley Periodicals, Inc.