I-LEVEL THEORY DEFINES PERSONALITY FACTORS AND RELATES THEM TO AN INDIVIDUAL MATURITY LEVEL. IT THEN STATES THAT SUCCESSFUL ATTEMPTS TO CHANGE BEHAVIOR MUST BE CONSISTENT WITH THE INDIVIDUAL MATURITY OR I-LEVEL. A TOTAL OF 1681 DELINQUENT BOYS AT THE PRESTON SCHOOL OF INDUSTRY, SACRAMENTO, CALIF., WERE DIVIDED INTO CONTROL AND I-LEVEL TREATMENT GROUPS. ANALYSIS OF PAROLE PERFORMANCE FOR 24 MONTHS FOLLOWING RELEASE FOUND THAT THE DIVISIONS OF THE INSTITUTION WHICH DID THE BEST JOB OF MATCHING TREATMENT TO I-LEVEL ALSO HAD THE BEST PAROLE PERFORMANCE. THE BEST MATCH WAS ACHIEVED IN THE PSYCHIATRIC UNIT. STATISTICAL METHODS USED TO MEASURE MATURITY LEVELS, DEVELOP TREATMENT METHODS, AND ANALYZE RESULTS ARE PRESENTED. IT IS SUGGESTED THAT A PREVIOUS STUDY OF THIS DATA WHICH CALLED THE I-LEVEL THEORY INVALID WAS BASED ON INAPPROPRIATE COMPARISONS. THE DIVISIONS OF THE SCHOOL WHICH SHOWED POOR PAROLE PERFORMANCE IN THE PREVIOUS STUDY WERE THE DIVISIONS WHICH HAD DONE A POOR JOB OF IDENTIFYING I-LEVEL AND DEVELOPING APPROPRIATE THERAPY.