Long-term follow-up in isolated ventricular septal defect considered too small to warrant operation. 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. 