Kala-azar (VL), mainly affecting infants and young children, is being increasingly reported in the south-west of Saudi Arabia, 305 cases being diagnosed in 1988. Most cases arise in scattered locations in the foothills west of the Asir mountains at altitudes between about 500 and 1000 m, although case clusters are apparent in some villages. Some cases also occur between the foothills and the Red Sea coast. The incidence in the south-west was calculated to be of the order of 6 to 8/10,000 population per year, but a random serological survey using the enzyme-linked immunosorbent assay and direct agglutination techniques indicated a seropositivity rate of about 3.7% in 706 apparently healthy children. Most patients are Saudi or Yemeni and cases are being increasingly identified also in the Yemen Arab Republic. In the foothills of both countries the causative organism is Leishmania donovani s.l. zymodeme LON-42, which also occurs on the eastern littoral of Ethiopia. By isoenzyme electrophoresis, it is readily separated from L. infantum, which has been identified in feral dogs, the 2 organisms being sympatric. The infection was found in 6.7% of 89 dogs, but their seropositivity rate was 19.3%. Although L. infantum has not yet been recognized in man in Saudi Arabia, it has been identified in a child in the coastal plain of the Yemen Arab Republic. Further research needed to provide a rational basis for control is discussed.