An isolated ventricular septal defect (VSD) was diagnosed in 70 patients (39 men and 31 women, mean age 29 years, range 10-64 years). Surgery was judged unnecessary. The follow-up period was at least 10 years, or until death or 31 December 1988, comprising a mean duration of 21 (range 6-29) years. The mortality was 11/69 (one lost to follow-up), and was not significantly higher than in a matched 'normal' group. Six deaths were cardiac, four of which could probably be related to the VSD. The follow-up study revealed that: (1) 14 (22%) subjects had major, VSD-related complications, and cardiac surgery was indicated in eight patients; (2) six (10%) had minor complications. By the end of 1988, 24% of subjects had significant dyspnoea, 22% had chest pain and 19% used cardioactive drugs. Only 33% were receiving regular cardiac control in a hospital. Thus unoperated adults with a small VSD should be monitored closely, since this condition is far from benign.