Global climate change is one of the instigating and contributing factors for epidemic outbreaks of infectious diseases in human populations. In the years 2003 to 2005 the city of Tampico, in the northern state of Tamaulipas, Mexico, experienced recurrent outbreaks of dengue virus infections (DV) and the resulting dengue fever (DF). One of the hallmark symptoms of DF, which appears to worsen as the environmental temperature increases, is thrombocytopenia. In as much as it is a hallmark for hemorrhagic manifestations, thrombocytopenia is a useful sign to monitor the course of infected patients. Extracellular calcium (Ca2+ o) plays a key role in blood clotting; its chelation in vitro with ethylenediaminetetracetic acid (EDTA) or citrate prevents clotting, while exogenous recalcification of plasma leads to shortening of clotting time. In vivo, Ca2+ o is essential for platelet function and for the regulation of the immune response. In this work we report a significant increase (p<0.05) in the number of blood platelets of patients with clinical signs and symptoms of DF following oral administration of calcium carbonate (CAL, 1.2 to 1.8 g/day; n=10) when compared with a control group (CTL, n=10): 89 (46-132) versus 206 (155-257). Data expressed as mean value (95% confidence interval, C.I.) for x1000 cells/mm3. CAL also improved overall clinical condition and reduced by 36 % the duration of signs and symptoms of DF: 6.7-11.3 days, versus 11.5-16.6 days (95 % C.I., p<0.05) when compared with CTL patients. The possible mechanism of calcium attenuated thrombocytopenia and clinical improvement is discussed.