The psychiatric morbidity among 718 Chinese primary school children from a lower middle social class area in Hong Kong was studied, using a two-stage design. The screening procedures included two parts, a multi-component scale and a battery of probing questions. The screening procedures achieved an overall sensitivity of 75% and a specificity of 97%. The findings on prevalence were: (i) a total prevalence of all definite psychiatric cases of 16.3%; (ii) prevalence according to diagnostic categories: emotional disorder, 8.8%; conduct disorder, 2.0%; mixed disturbance of emotion and conduct, 3.0%; hyperkinetic syndrome, 1.0%; hyperkinetic conduct disorder, 1.0%; and Gilles de la Tourette's syndrome, 0.4%; (iii) boys 2.7 times more affected than girls; (iv) emotional disorder much more common than conduct disorder, even in boys; and (v) the prevalence of borderline cases was 19.5%. The mean duration of morbidity for all definite cases was 41.7 months. About half of all definite cases were seriously impaired in one or more areas of functioning and the other half moderately impaired. These results were compared with findings obtained from studies done in other countries.