Three cohorts were selected for comparison: 222 individuals acquitted by reason of mental disease or defect between 1971 and 1976; 107 persons acquitted just prior to passage of the IDRA (1976-78) and followed for 18 months; and 110 individuals acquitted after passage of the IDRA (January to December, 1981) and followed for 18 months. The data suggest that the IDRA has had the effect of formalizing, juridicizing, accelerating, and extending to all acquittees a procedure that was to an extent in place prior to passage. Although more persons are reviewed under the IDRA and more are released at postacquittal hearings, of those who remain, fewer enter nonsecure facilities at commitment, fewer move to nonsecure facilities during the first 18 months of hospitalization, and fewer are released at 6-month review. Thus, the law appears to have tightened controls on persons designated as dangerously mentally ill. Results show that the IDRA did effect changes concordant with its goals of increasing public safety and addressing due process and equal protection issues raised in the courts. The IDRA and supplemental research materials and data are appended. 1 note and 51 references. (Author abstract modified)