To determine the frequency of gastrointestinal symptoms in diabetic patients, 190 patients, consecutively referred to the Diabetes Research Institute, reported their gastrointestinal symptoms on a standardized symptom list. One hundred and eighty non-diabetic healthy subjects served as (matched) controls. Finally, 75 patients with Type 1 (insulin-dependent) diabetes mellitus (33 male, 43 female; age 34,1 (18-60) yrs, diabetes duration: 11,1 (0,3-41) yrs) and 68 patients with Type 2 (non-insulin-dependent) diabetes mellitus (31 male, 37 female, age: 61,4 (37-88) yrs, diabetes duration: 10,7 (0,3-40) yrs) were studied and compared with two cohorts of controls of the same size. There were no differences in prevalence of symptoms referrable to the upper and lower GI-tract in type 1 diabetic patients as compared with controls. Among patients with type 2 diabetes the main gastrointestinal complaint was constipation (22,1% vs 10,3%; p < 0.05). Upper gastrointestinal symptoms were also more frequent among Type 2 diabetic subjects (nausea 11,8% vs 2,9%, p < 0.05). There was a tendency for an increased symptom prevalence with higher age in both type 1 and type 2 diabetes. Presence of peripheral neuropathy was associated with a higher symptom prevalence in type 1 diabetes. After stratification, diabetes duration and glycaemic control (HbA1c) did not influence the frequency of symptoms. Thus, gastrointestinal symptoms occur frequently among both diabetic patients and non-diabetic subjects. However, significant differences were found only in type 2 (non-insulin-dependent) diabetes, the commonest symptom being constipation. These findings support the need of a nondiabetic control group in epidemiological studies evaluating symptom prevalence in diabetes mellitus.