For this study, 683 only children seen in a psychiatric clinic were compared with 2364 children from two-child families seen in the same clinic. From the age of 5 years, there were no differences between the two groups. Among the under-5s, a higher proportion of only children had problems which did not warrant a psychiatric diagnosis, and their parents were more often rated as being over-protective. Our findings suggest, however, that the parents of only children differ from other parents not in being more over-protective but in being more willing or able to seek medical advice.