The incidence of Type 1 (insulin-dependent) diabetes mellitus was prospectively evaluated in Catalonia, Spain in patients up to 30 years of age during the period 1987-1990. The population at risk (0-29 years) consisted of 2,690,394 inhabitants (total population of Catalonia 5,978,638). All the cases were independently identified from four sources: endocrinologists, sales of blood glucose monitors and insulin pen injectors, diabetes societies and diabetic summer camps. The degree of ascertainment was 90.1%. The overall observed incidence rate was 10.7 per 100,000 per year, being 11.5 per 100,000 per year in the 0-14 age group. The incidence in males (12.0 per 100,000 per year) was higher than in females (9.3 per 100,000 per year), with a male/female ratio of 1.36/l. The sex differences were only present in cases over 14 years of age. Age specific incidence rates per 100,000 per year were 4.4 (confidence interval 95%: 3.2-5.7) in the age group 0-4, 9.9 (8.5-11.4) in 5-9, 17.5 (15.7-19.4) in 10-14, 11.4 (9.9-13.0) in 15-19, 11.3 (9.7-13.0) in 20-24 and 8.5 (7.2-9.9) in 25-29. There was a seasonal onset pattern, with the highest incidence in winter (December-February). We conclude that the incidence of Type 1 diabetes observed in Catalonia during the period 1987-1990 is higher than that recently reported in other Mediterranean countries. This study offers the first standardized data on Type 1 diabetes incidence in Catalonia, including cases up to 30 years, and contributes to the knowledge of the epidemiology of diabetes in South Europe.