The frequencies of pervasive developmental disorder (PDD) in Down's syndrome (DS) have been reported from 1% to 11%. However, it is not clear if the frequency of this co-occurrence is higher or lower than in other mental retardations. We study a large sample of DS population, finding a PDD frequency of 15.6%, with 5.58% of autism (eight males and two females) and 10.05% of PDD non autism (nine males and nine females. The meaning of this frequency is discussed.