A survey of 96 consecutive families whose Down’s syndrome index patients were born at a maternal age of less than 30 years, is reported. Seven cases of interchange trisomic Down’s syndrome were found, four of the D/G21 and three of the G21/G types; in two of the D/G21 and one of the G21/G cases, the interchange was inherited from a parent. In the remaining 89 families, the index patients proved to be examples of primary G21 trisomic Down’s syndrome. The mother of one of these carried a D/D interchange. The importance of chromosome studies for the genetic counselling of parents of Down’s syndrome children, is summarised.