We examined the prevalence, genotype-phenotype correlation, and natural history of lamin A/C gene (LMNA) mutations in subjects with dilated cardiomyopathy (DCM). Mutations in LMNAhave been found in patients with DCM with familial conduction defects and muscular dystrophy, but the clinical spectrum, prognosis, and clinical relevance of laminopathiesin DCM are unknown. A cohort of 49 nuclear families, 40 with familial DCM and 9 with sporadic DCM (269 subjects, 105 affected), was screened for mutations in LMNAusing denaturing high-performance liquid chromatography and sequence analysis. Bivariate analysis of clinical predictors of LMNAmutation carrier status and Kaplan-Meier survival analysis were performed. Mutations in LMNAwere detected in four families (8%), three with familial (R89L, 959delT, R377H) and one with sporadic DCM (S573L). There was significant phenotypic variability, but the presence of skeletal muscle involvement (p < 0.001), supraventricular arrhythmia (p = 0.003), conduction defects (p = 0.01), and “mildly” DCM (p = 0.006) were predictors of LMNAmutations. The LMNAmutation carriers had a significantly poorer cumulative survival compared with non-carrier DCM patients: event-free survival at the age of 45 years was 31% versus 75% in non-carriers. Mutations in LMNAcause a severe and progressive DCM in a relevant proportion of patients. Mutation screening should be considered in patients with DCM, in particular when clinical predictors of LMNAmutation are present, regardless of family history.