Background The Best Interest of the Child-Questionnaire (BIC-Q) has been designed as an instrument for screening the quality of the rearing situation of children with behavioural problems or delinquency. It is intended to aid legal decisions in juvenile and family law. Aims The aim of the study was to establish the reliability and validation of the BIC-Q. Methods Records of 83 children in detention or secure treatment centres and 58 children with no delinquent behaviours were rated by trained researchers. Interrater reliability was tested with a sub-sample of 31 of the former. Content validity was evaluated across the whole group, using Cronbach's alpha to check internal consistency, Spearman's rho to examine correlations and explorative principal component analysis (PCA) to test the extent to which variance in the 14 underlying conditions could be accounted for by a single factor 'quality of rearing'. Results Interrater reliability was good (r = 0.89). The PCA lent support to the content validity of the questionnaire with four factors, which together explained 66% of the variance in child rearing: (1) attachment and bonding; (2) rules and provisions; (3) health care and physical care in the family context; and (4) care and models provided for in the societal environment.