Analysis of data on the effects of passive smoking obtained in preadolescent children from the Harvard Six-Cities Study demonstrates an exposure-response relationship between the number of smokers in the household and the reporting rates for doctor-diagnosed respiratory illness before age 2, history of bronchitis, wheeze most days and nights apart from colds, and a composite of symptoms defined as the lower respiratory index. Similarly, when only the amount currently smoked by the mother was used, the data indicated a relatively uniform increase in each of the reported diseases and symptoms. FEV1 was lower in children with smoking mothers compared to children of nonsmoking mothers. Rate of increases in FEV1 after adjusting for normal growth was significantly smaller in children of smoking mothers and was related also to amount smoked. Notably the effect on level of FVC was not seen and this finding, consistent in several studies, remains unexplained. Although children of smoking mothers were shorter on the average than children of nonsmoking mothers, no on-going passive smoking effect on height growth can be ascertained. All these differences are small and their medical significance remains to be defined.