Heatwaves affect human health and should be more and more frequent because of global warming and could lead to increase mortality in general population, especially regarding cardiovascular mortality. During the summer 2019, Europe experienced a strong episode of heatwave. Telemonitoring of patients with heart failure (HF) provide an elegant tool to monitor closely the weights, and we assumed to be able to assess our hypothesis through a nationwide telemonitoring system. Here, we hypothesize that (i) there will be a change in patients' weight during the heatwave and (ii) that the telemonitoring would enable us to follow these changes. The change in weight would be a surrogate for clinical worsening (with or without decompensated HF). Briefly, 1420 patients with a median age of 73.0 years and mean weight of 78.1 kg have been included in this analysis. The relationship between temperature and weight is very strong (P < 10(-7) ). The magnitude of the effect seems clinically relevant with a variation of 1.5 kg during a short period. This could expose patients to increased symptoms, HF decompensations, and poor outcomes. These results suggest a new way to implement weight telemonitoring in HF. This suggests also a direct impact of global warming on Human health, with acute episodes that are expected to occur more often, threatening patients with chronic diseases, especially patients with heart failure. In clinical practice, this urges to take into consideration the episodes of extreme heatwave and suggest that we have already useful tools including telemonitoring available in frail patients.