This paper reviews findings on sex differences in health among children and adolescents, concentrating particularly on the ages 7-15 years. The overall picture is of a gradual emergence of excess morbidity in females over this life stage. In respect of overall levels of chronic illness, higher male rates in childhood are replaced by higher female rates which emerge in early-mid adolescence. This pattern is also seen in an examination of the rates of two common childhood conditions, asthma and migraine/headaches. An excess of psychological disturbance (largely neurotic disorders) among females emerges in early-mid adolescence, in comparison with younger ages where males predominate. Sex differences in the utilization of health services appear to mirror these patterns of excess morbidity. There is also evidence that with increasing age there is a strengthening in the sex patterning of illness behaviour. In contrast to the emphasis which has been given to explaining the emergence of a female excess in psychological disturbance and illness-related behaviours and attitudes during early adolescence, much less attention has been given to the understanding of the excess in physical complaints among females which also arises over this period. The paper suggests that this may be directly related to the relative lowering of their psychological well-being during early adolescence. Longitudinal studies are required in order to answer questions concerning age by sex interactions in respect of morbidity in childhood and adolescence.