Since 1990 in most Eastern European countries health care systems have been decentralized or are undergoing the processes of decentralization. Increasingly, diabetic patients are no longer treated by diabetologists but by non-specialized physicians. During the same period structured treatment and teaching programmes have been introduced and health care is increasingly influenced by the St. Vincent declaration. To show the effect of these changes on the quality of diabetes care 90 % (n = 244) of all insulin-treated diabetic patients aged 16 to 60 years and living in the city of Jena (100247 inhabitants) were studied in 1994/1995. The results were compared with the baseline examination of 1989/1990 (n = 190). HbA_1c) (HbA_1c/mean normal) in IDDM patients under specialized care was similar in 1994/1995 (1.54 ±0.27, n = 47) to 1989/1990 (1.52 ± 0.31, n = 131, p = 0.0018), but higher under non-specialized care (1.71 ± 0.38, n = 80, p = 0.0087). In the total group of NIDDM patients there was no significant change in HbA_1c (1994/1995: 1.75 ± 0.4, n = 117, vs 1989/1990:1.78 ± 0.4, n = 59, p = 0.67), but with a tendency to higher HbA_1c under non-specialized (1.81 ± 0.4, n = 79) compared to specialized care (1.66 ± 0.39, n = 38, p = 0.06). Incidence of severe hypoglycaemia (IDDM 0.13; NIDDM 0.04), ketoacidosis (0.02; 0.01) and the prevalence of nephropathy (21 %; 35 %) and neuropathy (24 %; 38 %) remained unchanged in comparison to 1989/1990, whereas there was an increase in the prevalence of diabetic retinopathy. Specialized care is mandatory for patients with IDDM. [Diabetologia (1997) 40: 1350-1357]