Genetic cardiomyopathies are complex diseases with heterogeneous clinical presentation and phenotypes. Early descriptions of cardiomyopathies originated from case studies involving individuals with severe, paradigmatic presentation, which provided insight into the worst-case scenarios of these conditions. With time, improved diagnostic sensitivity and awareness of cardiomyopathies has uncovered a more heterogeneous disease spectrum, including mild phenotypes overlapping with physiological variation. This diagnostic ‘grey area’ poses important dilemmas, particularly in athletes. Current screening policies have the potential to identify affected individuals at very early stages, leading to effective prevention of cardiomyopathy-related complications such as sudden cardiac death. Conversely, however, some physicians actively impose diagnoses on individuals who perceive themselves to be disease-free. In addition, the high sensitivity of contemporary diagnostic techniques carries a serious risk of misinterpreting physiological variation as disease. In this Review, three of the most common and controversial areas are discussed, including left ventricular hypertrophy; left ventricular dilatation, noncompaction, and fibrosis; and arrhythmias originating from the right ventricle. A systematic and cautious approach is necessary in patients with mild phenotypes suggestive of, but not definitely diagnostic for, cardiomyopathies. Preventing the mislabelling of healthy individuals and overdiagnosis should be a priority, with the aim to combine adequate counselling and optimal protection.