A reanalysis has been made of the data on blood pressure in a highland and a lowland village collected by the 1965-6 Expedition to the Simien Mountains of Ethiopia. Unlike other high altitude regions of the world, blood pressures among highland Ethiopians were found to be greater than among lowlanders. Socio-economic factors also appeared to be important; those of higher status having generally higher pressures than those of low status and exhibiting significant rises of diastolic pressure with age. The importance of chronic infectious disease as a cause of low pressure has been examined, in respect of malaria and respiratory disease, but no association was found. However, infectious disease is more widely prevalent at lower altitudes and it is tentatively suggested that this factor, in conjunction with socio-economic status, acts to produce the higher pressures seen among the high altitude dwellers studied.