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Seasonal variation in cardiovascular functions (CVFs) associated with climatic changes is an important emerging public health issue. The objectives of this study were to demonstrate seasonal variation in CVFs by comparing intra-individual differences between winter and summer among people working in a forest environment and to discuss the possible mechanisms accounting for the health effects of seasonal variation in cardiovascular hemodynamics. A total of 72 staff members of the Experimental Forest of National Taiwan University were recruited for continuous health monitoring during two seasons to investigate the intra-individual seasonal variation in CVFs, complete blood counts, and biochemical examinations. CVFs were assessed by measuring the arterial pressure waveform by a cuff sphygmomanometer using an oscillometric blood pressure device, and aortic stiffness was measured by brachial-ankle pulse wave velocity (baPWV). The results showed that cholesterol levels, white and red blood cell counts, and platelet counts were higher in winter than in summer. Subjects showed not only higher vascular stress, as indicated by higher levels of brachial systolic and diastolic blood pressure (SBP and DBP), central end-SBP and DBP, systemic vascular resistance (SVR), and baPWV, but also lower cardiac activities, including lower levels of heart rate, left ventricular contractility, and cardiac output in winter than in summer. The central and brachial BP, cardiac output, SVR, and baPWV were significantly associated with temperature changes in seasonal variation after controlling related confounding factors. This study provides evidence of higher vascular stress and susceptibility to atherothrombosis during winter compared with summer.