Sex differences in social attitudes have been well documented. Women hold more positive attitudes toward mental illness than men do. This paper reports on the effect of sex differences in a study of secondary school students' opinions about mental illness in Hong Kong. A total of 2,223 secondary school students, drawn by random sample, completed a 45-item questionnaire on Opinion about Mental Illness in Chinese Community (OMICC) with a six-point Likert Scale. Individual items with weak correlations were eliminated, leaving 33 items for analysis (Cronbach's Alpha = .866). Using factor analysis six factors were identified. These include: Benevolence, Separatism, Stereotyping, Restrictiveness, Pessimistic Prediction and Stigmatization. Results showed that girls scored higher regarding benevolence. Boys were found to have more stereotyping, restrictive, pessimistic and stigmatizing attitudes towards mental illness.