Background: Few studies have investigated the impact of air temperature on blood pressure (BP) and endothelial function. We examined short-term effects of air temperature on BP and endothelial function in two panel studies. We also investigated interactive effects between temperature and air pollution on endothelial function. Methods: Between March 2007 and December 2008, 371 BP measurements were conducted in 30 individuals with type-2 diabetes mellitus (T2D), 30 persons with impaired glucose tolerance and 42 individuals without a metabolic disorder from Augsburg, Germany. Hourly means of ambient meteorological data were obtained from a fixed measurement site. Minute by minute personal temperature measurements were conducted using data loggers. Between November 2004 and December 2005 flow-mediated dilatation (FMD) and nitroglycerin-mediated dilatation (NTGMD) were measured using brachial artery ultrasound on four consecutive days in 22 individuals with T2D in Chapel Hill, North Carolina (USA). Daily measurements of meteorological and air pollution parameters were obtained from fixed monitoring sites. Temperature effects were evaluated using confounder adjusted additive mixed models. Results: Systolic BP increased immediately by 0.8% (95% confidence interval:[0.4;1.1%]) in association with a 1°C temperature decrease on the same day in individuals with T2D in Augsburg. Effects of personally measured temperature were similar. A 1°C decrease in temperature was associated with immediate decreases in mean FMD (-2.2%[-4.7;0.3%]) and NTGMD (-1.7%[-3.3;-0.04%]) in Chapel Hill. Temperature effects on FMD strengthened when PM2.5 and ozone concentrations were high. Conclusions: We observed changes in BP and endothelial function in two panel studies in individuals with T2D indicating that these people might be potentially more susceptible to temperature changes. Both outcomes might be possible mechanisms explaining cardiovascular events in association with environmental stimuli.