OBJECTIVE - Early data have suggested a high prevalence of white coat hypertension (~50%) in NIDDM patients. To study this phenomenon further, we determined the prevalence of white coat hypertension in NIDDM patients with normo- or microalbuminuria or with diabetic nephropathy. RESEARCH DESIGN AND METHODS - Three groups of hypertensive NIDDM patients (repeated clinic blood pressure > 140/90 mmHg or antihypertensive treatment) attending the Steno Diabetes Center were investigated in a cross-sectional study. Group 1 had normoalbuminuria (a urinary albumin excretion [UAE] rate <30 mg/24 h, n = 30, age 61 ± 7 [mean ± SD] years, 20 men), group 2 had microalbuminuria (UAE rate 30-300 mg/24 h, n = 51, age 55 ± 7 years, 35 men), and group 3 had diabetic nephropathy (UAE rate >300 mg/24h, n = 47, 62 ± 7 years, 36 men). If given, all previous antihypertensive medication was withdrawn at least 2 weeks before the study (48%). The prevalence of white coat hypertension (clinic hypertension with normal blood pressure values at home) was determined by comparison of clinic blood pressure (Hawksley Random sphygmomanometer) and the ambulatory daytime (7:00 A.M. to 11:00 P.M.) blood pressure (A and D TM2420). By applying established criteria, white coat hypertension was confirmed if daytime blood pressure was <135/85 mmHg. RESULTS - The clinic blood pressure was 155/86 (SE 3/2) mmHg, 156/89 (2/1) mmHg, and 171/90 (3/2) mmHg in group 1, 2, and 3, respectively (P < 0.05 comparing group 3 with groups 1 and 2). The prevalence of white coat hypertension was significantly higher in group 1 as compared with groups 2 and 3, 23% (95% CI 10-42) vs. 8% (2-19) and 9% (2-20) (P < 0.05), with no difference between the latter two groups. CONCLUSIONS - The prevalence of white coat hypertension in normoalbuminuric NIDDM patients resembles that observed in nondiabetic subjects with essential hypertension, whereas the prevalence is significantly lower in NIDDM patients with incipient or overt diabetic nephropathy, suggesting a difference between primary and secondary hypertension.