OBJECTIVE(S) Twenty-four-hour ambulatory blood pressure monitoring (ABPM) seems to be the most accurate way of diagnosing hypertension in patients with obstructive sleep apnea (OSA). However, this diagnostic tool is expensive and time-consuming and is therefore not used routinely. We aimed to find baseline predictors that enable the identification of patients who misclassify themselves as nonhypertensive to optimize the use of ABPM. METHODS Clinically suspected OSA patients (n=369) were enrolled and underwent overnight polysomnography and 24-h ABPM, and completed a data collection form. Anthropometric measurements were assessed. Generalized additive models, the minimum P-value approach, and logistic regression models were used for data analysis. Results were considered significant when α is equal to 0.05. RESULTS One hundred and twenty-two patients who were not on antihypertensive medication did not report hypertension, but according to ABPM data, 43.4% (n=53) of them had misclassified themselves as nonhypertensive. These patients had a significantly higher apnea-hypopnea index (P<0.001), ABPM variables (P<0.001), BMI (P=0.002), and neck circumference (NC) (P=0.002) than nonhypertensive patients (n=69). BMI and NC emerged as independent predictors of hypertension misclassification. The cut-off points that best discriminated the two groups of patients were 27 kg/m and 39 cm for BMI and NC, respectively. The resulting binary BMI and NC remained independent predictors of hypertension misclassification in the final model (odds ratio: 3.2, P=0.010; odds ratio: 2.4, P=0.038). CONCLUSION Our findings emphasize the importance of ABPM for the diagnosis of hypertension in patients suspected of having OSA with a BMI and NC above 27 kg/m and 39 cm, respectively.