This article describes a study of summertime classroom temperature and its relationship to the clothing worn by English secondary school children. Data on classroom temperatures and children's clothing were recorded during one summer for a total of 987 lessons. Analysis of the data showed that the strong clothing-temperature correlation could be used to deduce the proportion of children in thermal comfort. The clothing observations were supplemented by a more limited conventional comfort study. These subjective data agreed well with the results that had been deduced from the clothing observations. A comparison was also made with other subjective work and with predictions made on the basis of heat exchange theory. Consideration of the results suggests that for thermal comfort classroom temperatures should rarely exceed 26 degrees Centigrade and that departures from the prevailing mean temperature should not normally exceed 2 degrees Centigrade. For children in shirtsleeves or summer dresses, the optimum temperature was 24.5 degrees Centigrade, and for children wearing jumpers or blazers, it was 21.5 degrees Centigrade. (Author/JG)