In order to assess the relative and interactive importance of genetic and environmental components on the development of myopia in Chinese school children aged from 10 to 15 years, a population-based sample of 361 same-sexed twin pairs recruited through stratified cluster sampling was studied. Zygosity of twin pairs was determined by Mendelian traits, red cell antigen systems, and continuous dermatoglyphic characteristics; while myopia was diagnosed by both objective and subjective techniques. Studying and reading habit was obtained from cotwins and their parents through a life style questionnaire. Age-sex-adjusted concordance rate derived from multiple regression equation was used in the analysis. Conventional comparison of intrapair concordance between monozygotic (MZ) and dizygotic (DZ) twins was used to assess the importance of a genetic component in the determining of myopia, and a significant genetic influence was observed. Environmental influence on myopia was evaluated through MZ cotwin method, and MZ cotwins with concordant studying and reading habits were significantly more concordant in myopia than those MZ cotwins with discordant habits. The possible effect of gene-environment interaction on myopia was explored, and concordance in myopia was found significantly associated with the interaction between zygosity and habit of studying and reading. These observations suggested that the impact of the environmental factor on the development of myopia may be influenced by genotype, and vice versa.