The possible association between heart size measured during a cardiovascular screening examination and cardiovascular mortality was studied in 1984 healthy men aged 40-59 years. At the 16-year follow-up 278 had died, 150 from cardiovascular diseases. Cardiovascular mortality was 2.2 times higher among the 122 men with heart size > or = 500 ml.m-2 than among those with heart size < 500 ml.m-2. This association was, however, exclusively confined to men with physical fitness below median in whom the corresponding mortality ratio was 4.6 (95% confidence interval 2.5-8.4; P < 0.001) after adjustment for age, smoking, cholesterol, blood pressure and heart rate. Heart size measurements from routine chest X-rays is fast, easy, inexpensive and appears to provide valuable, independent screening information in healthy, middle-aged men.