This study examined the relationship between the severity of periodontal disease and organ complications in long-term Type 1 or insulin-dependent diabetes mellitus patients, taking account of the severity and concomitant existence of these complications. The population studied consisted of 26 Type 1 diabetics 26 to 34 years old, who had had diabetes for at least 10 years. Severity of periodontal disease was shown to increase with severity of organ complications. Patients with advanced complications had significantly more bleeding on probing, pockets > or = 4 mm deep, and more attachment loss than patients with incipient complications or no complications. Stepwise multiple regression analysis showed that the existence of advanced complications was the only diabetes-related factor predicting pockets > or = 4 mm deep. Subgingival calculus, sex, and smoking were other significant variables. Retinopathy was the organ complication most suited to comparison, since it is usually the first to appear and can easily be classified from non-existent to severe. Differences in severity of periodontal disease were less obvious if metabolic balance alone was considered than between subgroups formed on the basis of the existence of advanced complications. Severity of periodontal disease and the existence of complications were more closely related to long-term glucose balance than single, most recent HbA1 values. Prevalence of pockets at sites with subgingival calculus increased with severity of complications.(ABSTRACT TRUNCATED AT 250 WORDS)