DURING the past decade, the Socio-Behavioral Laboratory of Pacific State Hospital examined several characteristics of hospitalized mentally retarded patients, and the natural history of their hospitalization.1-27Some studies were based on groups of newly admitted patients (admission cohorts),1,2,4-7,9,13,15,17-20,24,25,27others involved cross sections of populations.3,8,10-12,14,23,26Some focused on releases from the hospital,3,4,8,10,11,14others on mortality,5,15,27or both.1,2,9,16The changes in the composition of the patient population,18,23the patterns of release,20the effects of admission procedures6,7,12,13,18,21,22,24and the efficacy of selected treatment programs19,25,26were also evaluated. In most studies, new or modified methods of data analyses and related mathematical models were presented. In 1957 and 1958 we reported on a four-year (1948 to 1952) admission cohort1,2and related probabilities of release and death to age, intelligence quotient, and diagnosis. We found that: (1) younger and more severely retarded patients had a