MODY3 diabetes, which is caused by a mutation in the hepatocyte nuclear factor-1alpha gene (HNF-1alpha) on chromosome 12, represents a relatively common monogenic form of diabetes in Finland. Age at onset of the disease can vary from 10 to 60 years, but little is known about the natural course of the disease, particularly the development of diabetes-related chronic complications. The availability of genetic markers now allows description of the clinical course of the disease. In order to examine the prevalence of chronic diabetic complications in MODY3, we examined 57 carriers with HNF-1alpha mutations for the presence of micro- and macrovascular complications. Thirty-four percent of the MODY patients had mild and 13% had severe non-proliferative or proliferative retinopathy; this figure did not differ from the figures in insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM) patients matched for duration and glycaemic control but not for age. Neither did the prevalence of microalbuminuria differ between MODY3 and IDDM or NIDDM patients (19 vs 24 and 23%). Neuropathy was observed with the same frequency as previously reported in IDDM. Hypertension was less frequent in MODY3 and IDDM than in NIDDM (24.5 and 19 vs 53.7%; p < 0.001). Coronary heart disease was more common in MODY3 than in IDDM (16 vs 4.5%; p < 0.02) but less common than in the older NIDDM patients (33.3%; p < 0.02). In a multiple logistic regression analysis, poor glycaemic control was an independent risk factor for retinopathy (p = 0.03), microalbuminuria (p < 0.04) and neuropathy (p = 0.03). In conclusion, microangiopathic complications are observed with the same frequency in patients with MODY3 diabetes as in IDDM and NIDDM and are strongly related to poor glycaemic control.