We present a Norwegian family, followed since 1967, with a chromosome 6q24 duplication in two siblings with neonatal diabetes, in their non-diabetic father, and in a female (third generation) with adult-onset diabetes. The parents (first generation) were healthy and non-consanguineous. After a miscarriage, the couple had two infants with birth weights of 1780 and 1620 g, respectively, both of whom died on their second day of life. Patient I (male, weight 1840 g at term) had a blood glucose level of 33 mmol/L on day 6. He was treated with insulin for 3 months. In adult life he had permanent diabetes, treated with oral hypoglycemic agents. At 43 yr of age, there were no diabetic late complications. Patient II (female, birth weight 1440 g at term) had an increasing blood glucose of 55 mmol/L on day 13. She received insulin treatment for 12.5 months. Subsequently, she was successfully treated with sulfonylurea (tolbutamide) for 10 yr. At 11 yr of age, insulin was again considered necessary. At 40 yr of age, no diabetic late complications were detected. Patient III had a birth weight of 2630 g at term and no diabetic symptoms as a neonate. She had insulin-requiring diabetes from age 19. We conclude that (i) neonatal diabetes with chromosome 6q24 duplications may become a permanent disease in adult life; (ii) this chromosome anomaly may also be associated with adult-onset diabetes; (iii) sulfonylurea treatment may be attempted, and (iv) late diabetic complications may be absent, even after more than 40 yr.