Available data on the impact of certain types of families is lacking, and the results are often misleading with respect to maladjustment. Following a description of variations in delinquent activity and behaviour problems according to family type, the authors analyse the difficulties in the operation of family systems. Comparisons of six family types apply to data from 763 boys aged 10, 319 female and 426 male adolescents, aged 14 and 15: intact families, father-based and mother-based single-parent families, father-based and mother-based reconstituted families and substitute families. The article's data show that in the late eighties, nearly 40 per cent of children and adolescents living in low-income districts in Montreal belonged to disunited families. In addition, the data confirm a classic observation: in comparison with intact families, disunited families are underprivileged in relation to living conditions, deficient in relation to psychosocial functioning, and propitious to behaviour problems and delinquent activity. In addition, it has been established that certain disunited family types represent a considerable risk factor. The damaging effect of family structure increases in the following order: intact families, mother-based single-parent families, mother-based reconstituted families, substitute families, father-based reconstituted families and father-based single-parent families. Finally, certain intervention methods are suggested to help prevent behaviour and family problems.