The past half-century has seen significant changes in family life, including an increase in parental divorce, increases in the numbers of lone parent and stepfamilies, changes in socioeconomic well being, and a decrease in family size. Evidence also shows substantial time trends in adolescent mental health, including a marked increase in conduct problems over the last 25 years of the 20th Century in the UK. The aim of this study was to examine how these two sets of trends may be related. To illustrate the complexity of the issues involved, we focused on three well-established family risks for conduct problems: family type, income and family size. Three community samples of adolescents from England, Scotland and Wales were compared: 10,348 16-year olds assessed in 1974 as part of the National Child Development Study, 7234 16-year olds assessed in 1986 as part of the British Cohort Study, and 860 15-year olds assessed in the 1999 British Child and Adolescent Mental Health Survey. Parents completed comparable ratings of conduct problems in each survey and provided information on family type, income and size. Findings highlight important variations in both the prevalence of these family variables and their associations with conduct problems over time, underscoring the complex conceptual issues involved in testing causes of trends in mental health.