In order to precise the prevalence and incidence of IDDM in a well characterized population, we have ascertained between 1979 and 1988 all juvenile diabetic patients living in the department of Oran (Wilaya of Oran), Algeria, aged less than 20 years at the date of Dec. 31th 1988 who had onset of the disease before 15 years old. Cross comparisons of the data obtained both by a IDDM registry and a questionnaire sent to the practitioners show an excellent correlation. A total of 173 families was studied, including 155 Simplex Families (SF), and 18 Multiplex Families (MF). The prevalence was 0.27 p. 1000 among children aged less than 15 years. The annual incidence (mean = 4.4 p. 100,000) increased regularly from 1.6 p. 100,000 in 1981 to 8.1 p. 100,000 in 1988. The prevalence of IDDM among siblings was 3.8%. The birth order of index cases (first affected siblings) was lower in MF (3.0 +/- 0.4) than both in index cases of SF (4.0 +/- 0.3; p less than 0.01) and in secondary affected siblings of MF (4.8 +/- 0.05; p less than 0.001). The frequency of 2nd degree consanguineous marriages (between first cousins) was greater among the IDDM families compared with control families: 25% vs 11% (RR = 2.9; p less than 0.001).