Comparable methods were used to study the rates of behavioural deviance and psychiatric disorder in ten-year-old children living in an inner London borough (ILB) and on the Isle of Wight (IOW). The total population in this age group was screened using a teacher's questionnaire of known reliability and validity, and a sample of this group (chosen partly at random and partly because of deviant scores on the questionnaire) was studied intensively to provide a psychiatric diagnosis. It was found that the rate of deviance (as assessed by questionnaire) and of psychiatric disorder (as assessed from parental interview) was twice as high in ILB children as in IOW children. The higher rate applied to both boys and girls and extended to both emotional disorders and disturbances of conduct. A systematic series of checks was initiated to determine whether the difference in rates could be due to any form of bias or lack of instrument validity, and it was concluded that it could not be. There was a true excess of behavioural deviance and psychiatric disorder in ILB children. A questionnaire comparison between the ILB studied and other parts of inner London showed that the high rate of disorder applied to inner London as a whole (although it varied somewhat from division to division) and was not specific to the borough studied. This paper constitutes the first of a series reporting an investigation of attainment and adjustment in two geographical areas. The next paper (Berger, Yule and Rutter) reports the findings for specific reading retardation and the overlap between this and psychiatric disorder. The third paper (Rutter et al.) considers some possible reasons for the difference in rates of deviance and disorder in the two populations.