The relationship between measures of mental ability, symbolic play and expressive and receptive language was investigated in a sample of 73 children with Down's syndrome (mean CA 48.9 months, range 19-90 months; mean MA 30.4 months, range 13-73 months). Language delay became increasingly apparent with age, and more so in boys than girls. There was no significant difference between mental ages derived from standard ability tests and corresponding age equivalent scores of the Lowe and Costello symbolic play test. A developmental progression of play similar to that seen in non-handicapped groups was found. However, it is suggested that the use of a more detailed scoring system would be informative.