Based on the register of all persons registered in the Danish National Service for the Mentally Retarded (DNSMR) with a diagnosis of Down's syndrome in the period 1 January 1976-31 December 1980, the persons were followed until 31 March 1984 (a total of 2412 Down's syndrome patients). The difference in mortality for patients in institutions and living outside was significant, with a higher mortality for women with residence in institutions than women living outside. Among males the mortality rate was not significantly different between the two types of residents. Of the total group, a survival rate of 82.4 was found at the end of the age group 5-9 years and this result is similar to recent results from Salford and Japan. Although the life expectancy for the Down's syndrome group has increased, the life expectancy for Down's syndrome at any given age compared with life expectancy for the background population during the above mentioned period was still lower.