During a period of eight years from 1977, pupils in F primary school in S ward, Tokyo were monitored each year with a general medical check-up to identify and eliminate any children with urine abnormalities and then the hydroxyproline: creatinine ratio (HOP ratio) in the urine of 4,375 children chosen at random was measured. The children's area of residence was classified, according to distance, into three areas. In addition, the children were classified into four groups depending on their estimated degree of exposure to Environmental Tobacco Smoke (ETS). The relationship between ETS and the HOP ratio was observed annually. The HOP ratio was greatest in schoolchildren living in the area nearest the main roads. The influence of ETS also tended to increase when ventilation rates in their rooms were lowered. The strength of these associations was considered significant depending on the magnitude of the HOP ratio, the correlation coefficient, a two-way layout and relative risk (R.R.). In addition, the consistency and coherence observed in measurements each year suggested a causal relationship. The influence of ETS and area distinctions on the HOP ratio was totally independent and showed virtually no interaction. However, the changes in patterns of indoor smoking from 1980 onwards were not sufficiently accounted for by the previous interview and questionnaire surveys so we introduced an additional bias, namely misclassification of pupils. Subsequently, the possibility of underestimation of the influence of ETS was considered, thereby establishing the need for a revision of the relevant questionnaires.