In many countries, the highest all-cause (including cardiovascular) morbidity and mortality were observed during winter. Seasonal and monthly trends in blood pressure (BP) may be one of the mechanisms of this phenomenon. Objective -to estimate seasonal and monthly trends in summer and winter clinical BP (CBP) and ambulatory BP (ABP) in patients with hypertension and in those with high normal BP (HNBP) in two regions of the Russian Federation, which have different climatic characteristics. Patients and methods. The investigation enrolled outpatients from Ivanovo and Saratov (i.e. the towns differing in average annual and average monthly temperatures). They were aged 40-79 years and had CBP <160/100 mm Hg in the presence and absence of regular antihypertensive therapy (AHT) or CBP 130/85-139/89 mm Hg in the absence of AHT without acute and uncompensated chronic comorbidity. All the participants signed an informed consent form. The patients were uniformly included in winter (December-February) and summer (June-August). In all the participants, CBP was measured using an automatic BP device (OMRON 705 IT); ABP was studied through 24-hour BP monitoring (BPM) with a BPLab device (OOO "Petr Telegin", Nizhny Novgorod, Russia), and standard examination was performed. Statistical analysis. Descriptive statistics and analyses of the means, standard deviations, errors, and frequencies of the examined parameters were used. The single-factor analysis of variance (ANOVA) was applied to estimate the significance of differences between the groups; the differences between the categorial variables were assessed via the Pearson's chi-squared test. Results. The survey covered 1630 patients: 720 and 910 people in Ivanovo and Saratov, respectively. In the patients from Ivanovo, the region with lower average temperature,than in those from Saratov, the mean clinical systolic BP (CSBP) was higher in winter than that in summer (134.4±13.2 and 129.7±12.6 mm Hg, respectively; p<0.0001). In the Saratov patients, CSBP in winter was also higher (128.9±14.3 mm Hg) than that in summer (125.0±16.0 mm Hg; p<0.001). The seasonal trend in clinical diastolic BP (CDBP) and the seasonal differences in average daily and daytime ambulatory SBP (ASBP) and ambulatory DBP (ADPM) were similar in the patients from both regions (p<0.05), except average daily SBP in the Ivanovo patients (p>0.05). In the latter group having normal CPB, average daily and average daytime BP values were elevated in particular in winter. In the Ivanovo patients, nighttime ASBP proved to be significantly higher in summer than that in winter (119.0±14.4 and 121.9±15.3 mm Hg, respectively; p<0.05) and to be greater than the reference values. In the Saratov patients, average nighttime SBP and DBP did not differ in winter and summer. There were no differences in average in winter and summer nighttime SBP and DBP between the patients from the two regions. In the examined cohorts, the clinical and ambulatory heart rate (CHR and AHR) is unrelated to a season. The monthly ABP and AHR changes did not demonstrate the clear trend characteristic for the average values. There was no relationship between average monthly CBP/ABP values and temperatures in summer and winter. Conclusion. In the patients with hypertension and in those with HNBP, the seasonal trend in CBP/ABP generally corresponds to the known patterns: BP is higher in winter than in summer. In the relatively colder region, winter daytime and summer nighttime ABP values were greater than the reference ones despite the normal CBP. The absence of significant interregional differences in nighttime ABP appears to be associated with the examinees' nocturnal sleep at nearly equal indoor temperature in both winter and summer. The causes of higher SBP in the Ivanovo patients during summer may be associated with the absence of AHT or its decreased volume or higher and less comfortable indoor temperature in summer. These findings suggest that there is a need for further investigations in this area, including for the assessment of the contribution of seasonal factors to the long-term prognosis of hypertensive patients.