The aim was to describe the natural history of adults with hereditary muscular dystrophies, including myotonic dystrophy, with respect to muscular function, ventilation and electrocardiogram. In a prospective study, 46 subjects were followed over a period of five years. In 1991 and 1996, their muscle function was assessed according to an observation scheme and their lung vital capacity was measured by spirometer. Electrocardiograms were obtained in 1991, 1993 and 1996. Deterioration of muscular function was seen with regard to both the functional muscle tests and the vital capacity. The proportion of pathological electrocardiograms increased from 38% in 1991 to 54% in 1996 in the 26 patients with myotonic dystrophy without an increase in clinically detected cardiac abnormalities. Timely examinations using standard methods can reveal medically important information on deterioration, which often passes clinically unnoticed because of the insidious progress of the diseases.