THE EFFECTS OF A PRESCHOOL INTERVENTION PROJECT ON OLDER BROTHERS AND SISTERS OF THE EXPERIMENTAL SUBJECTS WERE INVESTIGATED. DURING THE INTERVENTION RESEARCH, CONCERN HAD BEEN EXPRESSED FOR POTENTIAL DETRIMENTAL EFFECTS ACCRUING TO THE OLDER SIBLINGS THROUGH INVIDIOUS COMPARISONS WHICH MIGHT BE MADE BY SCHOOL PERSONNEL AND PARENTS. OLDER SIBLINGS IN THE ADJACENT BIRTH ORDER, REMOVED NO FURTHER THAN 2 YEARS FROM THE EXPERIMENTAL CHILDREN, WERE SELECTED FOR STUDY. A NUMBER OF INSTRUMENTS WERE USED TO OBTAIN DATA RELATED TO BOTH THE EXPERIMENTAL CHILDREN AND THE OLDER SIBLINGS ON VARIABLES OF SCHOOL ACHIEVEMENT, PERSONAL ADEQUACY, CLASSROOM ADJUSTMENTS, AND FAMILY RELATIONS. IN THE ANALYSIS OF THE DATA GAINED, LITTLE EVIDENCE WAS FOUND TO SUPPORT THE ORIGINAL CONTENTION THAT OLDER SIBLINGS OF CHILDREN INCLUDED IN AN EXPERIMENTAL PRESCHOOL PROGRAM SUFFER ADVERSELY IN ACADEMIC ACHIEVEMENT, PERSONAL DEVELOPMENT, OR SOCIAL DEVELOPMENT BY SUPPOSED OR ACTUAL INVIDIOUS COMPARISONS. (JH)