Relationships between blood pressure (BP) and environmental temperature were investigated as part of a broader study of constitutional and environmental determinants of BP in a sample of 1037 9-year-old Australian children. A Dinamap semi-automatic device was used to obtain three BP readings for each child. Average systolic blood pressure (SBP) and diastolic blood pressure (DBP) differed according to month of examination, with higher levels in the colder months. Negative relationships were observed between BP and temperature on the measurement day; computed regression equations indicated that a rise in maximum daily temperature of 10 degrees C was associated with falls of 5-7 mmHg in SBP and DBP. The relationships were independent of age, weight, height, socio-economic status and heart rate (HR). The results emphasize the importance of taking environmental temperatures into account in epidemiological studies of BP.