This bulletin presents findings from the Program of Research on Causes and Correlates of Delinquency. The co-occurrence or overlap of serious delinquency with drug use, problems in school, and mental health are discussed using data from the first 3 years of the program. Youth who are seriously delinquent often experience difficulties in other areas of life, and with the exception of the co-occurrence of drug use and delinquency, little is known about the overlap of these problem behaviors among youth. Specifically, co-occurrence of serious delinquency is looked at with school behavior; drug use; mental health; and the combination of these behaviors. Preliminary findings show that a large proportion of serious delinquents are not involved in persistent drug use, nor do they have persistent school or mental health problems. The problem that occurs most frequently is the use of drugs, and for males, as the number of problem behaviors increase, so does the likelihood that an individual will be a serious delinquent. Consequently, it is important to identify and address unique needs of individual youth rather than proceed with the assumption that all offenders require similar treatment. (Contains 2 figures, 8 tables, and 24 references.) (JDM)