The Leishmanin skin test was performed on 1353 people in a kala-azar endemic region of south-west Ethiopia. Physical examinations were also carried out on 2723. Two of these individuals, both males, had active visceral leishmaniasis with Leishmania organisms demonstrated by spleen puncture. Two other males, including one member of the research team, had parasitologically proven cutaneous leishmaniasis. Because there was negligible migration and little movement of individuals outside of their tribal territories, the geographical distribution of skin test positivity and clinical findings could be determined and correlated with environmental parameters. The level of positive skin tests for the groups tested ranged from over 64% for the three tribes collectively inhabiting the lower regions of the Omo Valley (altitude approx. 500 m) to 6.4% for the Suri tribe, which lives at 1400 m. Skin test positivity was highest in areas of deeply fissuring montmorillonite soils and where Phlebotomus langeroni orientalis have been collected. Termite mounds of the pipe-organ type seemed to occur independently of the proportion of positive skin tests, possibly because alternative resting and breeding sites for sandflies were available in the cotton clay soil or because of the cultural patterns of the people. Almost always, males had a markedly higher prevalence of positive skin tests than did females. The degree of positivity was strongly correlated with increasing age, most positive conversions occurring in the ten to 20 year olds, the age at which males join cattle camps as part of their herding activities. Splenomegaly reached a prevalence of nearly 50% among the Hamar speaking people to the east of the Omo River, where the pattern of disease suggests malaria as the principal cause. Hepatomegaly, however, was highest in the lower Omo Basin among the Nyangatom, Dassanetch and Kerre, where hydatid disease was a major cause of liver enlargement, but seemed unrelated to the proportion of positive Leishmanin skin tests.