OBJECTIVE: Diabetes or impaired glucose tolerance are common in patients with Cushing's syndrome (CS). In this study we investigated the incidence of subclinical Cushing's syndrome (SCS) in type 2 diabetic patients with poor and good metabolic control and the relationship between blood glucose and cortisol levels. DESIGN: A total of 400 type 2 diabetes mellitus (T2DM) patients (Group A, HbA1c >= 8%, n=250; Group B, HbA1c <= 6.5%, n=150) were included in the study. Biochemical tests followed by the 1 mg dexamethasone suppression test (DST) were performed. If post-DST cortisol levels were above 1.8 mu g/dl (non-suppressed group), the 2 mg DST test was performed for 2 days. Among the patients, post-DST cortisol of at least 1.8 mu g/dL and midnight serum cortisol levels of at least 7.5 mu g/dL confirmed the diagnosis of CS. RESULTS: While SCS was observed in 5 patients (2%) in Group A, no case was observed in group B. There was a statistically significant difference between groups for basal cortisol and post-DST cortisol results (p<0.001 and p<0.001, respectively). Microvascular complication rates were higher in the non-suppressed group (p=0.007). Post-DST cortisol levels had a positive correlation with the number of complications and HbA1c levels (respectively r=0.213, p<0.001, and r=0.191, p<0.001). Multivariate regression analysis revealed that durations of DM, HbA1c level, and post 1 mg DST cortisol levels were associated with the number of complications. CONCLUSIONS: This study shows the presence of SCS in a notable number in T2DM patients having poor metabolic control. Screening with 1 mg and 2 mg DST of T2DM patients with poor metabolic control who are also obese and hypertensive and have microvascular complications may be an appropriate method for detection of SCS.