Yaws (buba, Frambösie, pian), a non-venereal treponematosis of skin and bones is a tropical disease that affects primarily children. Eradication programs launched between 1950 und 1970 reduced the incidence substantially; resurgences, however, have been reported from several countries. We have studied a geographically isolated focus of yaws in the Ubangi area of northwestern Zaire. In survey I, in 1981, of the 4.407 participants from 11 villages, 348 (7.9%) had active lesions of yaws. All patients and their contacts were treated immediately with benzathine penicillin G. In survey II, in 1982, in 6 of the previously studied villages, of 5,390 participants, 136 (2.5%) had active lesions of yaws. In 1981, the predominance of primary lesions (86%) and high numbers of patients in all age groups, with the highest prevalence (23.4%) in children 10-14 years of age, suggest that yaws recently recurred at this focus. In 1982, only 25.7% of the lesions were primary and the highest prevalence (6.5%) had shifted to the younger age group of 5-10 years-old. In survey I more males (63%) were infected in the age group 0-24 years, and more females (89%) in those over 40 years. In survey II, more males (61.3%) were infected in the age group 0-14 years, and only females (100%) in the age groups 25 years and older. Seventy-one percent (1981) and 86% (1982) of the initial lesions were on the lower limbs. Participation of the population in survey I was 41.7% and was greatest (62%) in the locality where there was a newly established dispensary. In survey II, participation rose to 73.8%. Areas adjacent to the focus of yaws with longstanding dispensaries were free of yaws.(ABSTRACT TRUNCATED AT 250 WORDS)