Difficult-to-control arterial hypertension is a common medical problem that may result from severe hypertensive disease or from poor adherence to the recommended medical treatment. The identification of non-adherent patients is challenging, especially when non-adherence is intentional. The current report describes the use of serum levels of prescribed antihypertensive drugs to evaluate the adherence in individuals with difficult-to-control arterial hypertension. Serum drug levels (SDLs) were evaluated by liquid chromatography with mass spectrometry. The chromatographic separation was performed on a reversed-phase column with a gradient flow of the mobile phase. The detection of analyzed substances was accomplished on a linear ion-trap mass spectrometer. The subjects were labeled as non-adherent when the serum level of at least one of the evaluated drugs was below the limit of quantification. The study used data from 84 patients with arterial hypertension who underwent SDL assessment to verify compliance with the recommended treatment. Patients who presented with uncontrolled blood pressure despite the recommended combination of at least three antihypertensives were enrolled in the analysis. Based on the evaluation of the SDLs, all of the evaluated drugs were found in the sera of 29 (34.5%) of the study patients. In the remaining 55 (65.5%) patients, non-adherence was diagnosed. None of the prescribed antihypertensive drugs was detected in the sera of the 29 (34.5%) patients. Our data suggest that an assessment of SDLs might be helpful before an extensive evaluation is initiated for difficult-to-control hypertension.