Endemic elephantiasis of the lower legs in Ethiopia, which reaches a maximum of 86-7 per 1,000 adults in affected areas, is related to the distribution of red clay soil derived from volcanic rocks, particularly basalt. Prevalence falls rapidly on leaving these areas. This observation has been tested in regions of non-filarial elephantiasis reported in Kanya and north-western Tanzania and further investigated in volcanic areas of Rwanda where the disease had not previously been reported. The same relationship is found to occur in these areas. The limitation to the lower legs of the barefooted section of the farming community suggests that the aetiological factor or factors enter by the feet. The occurrence at high altitude (over 1,200 metres) is noted and the predominance of basalt or basalt-like lava in each case is considered significant. The altitude governs rainfall and temperature and thus governs the type of soil produced. The soil produced from these rocks is rich in colloidal iron oxide, alumina and silica, to which a number of metallic ions are adsorbed. This soil is a reddish-brown clay which, when wet, is strongly adherent to the skin. The derived ions are known to be toxic to human tissue and absorption through intact human skin has been shown to occur experimentally. It is suggested that absorption of these irritants through the bare feet is responsible for the irreversible damage to the lymphatic channels. The present studies support the hypothesis that "high-altitude" elephantiasis of the lower legs in East Africa is a geochemical disease.