Data from 6 States for the years 1968 and 1978 were used to gather information on random samples of (1) approximately 400 adult male admittees to State prisons and 400 adult male admittees to State mental hospitals in New York and California and (2) on 300 admittees to State prisons and 100 admittees to State mental hospitals in Arizona, Texas, Iowa and Massachusetts. Data were gathered on the person's history of arrest, imprisonment, and mental hospitalization. Data analysis indicates that considerable deinstitutionalization of State mental hospitals occurred in all six study jurisdictions. However, there is little consistency in the percentage of prison admittees with a history of at least one prior mental hospitalization. In New York, Arizona, and Massachusetts, the percentage of admittees with prior hospitalization decreased between 1968 and 1978. California, Texas, and Iowa, on the other hand, recorded significant increases in these percentages. Conclusions indicate that there is little support for the hypothesis that prisons and mental hospitals are functionally interdependent. The source of the explosion in the U.S. prison population apparently must be found elsewhere than in the deinstitutionalization of mental patients. One plausible rival hypothesis is that increases in the population at risk of committing crime led to an increase in the rate of serious crimes punishable by imprisonment. Tabular data and 32 reference notes are provided.