BACKGROUND: Heat waves have been reported as being associated with increased rates of hospitalizations and deaths. MATERIALS AND METHODS: In July 2011, a heat wave hit southern Italy. We enrolled 9,282 consecutive patients who called the Apulia (southeastern Italy) regional free public emergency medical service (EMS) "118" number (out of 4 million inhabitants) during July 2011. All patients were evaluated with a prehospital electrocardiogram (ECG) thanks to telecardiology support provided by a single telemedicine hub. Local temperatures and relative humidity were recorded and combined in order to calculate the heat index (HI), a more accurate parameter to assess perceived discomfort caused by hot temperatures. RESULTS: The mean number of calls to the telecardiology hub for prehospital ECG screening in the case of suspected heart disease was increased 48 h after days with an HI >/= 44 (402 +/- 68 versus 275 +/- 52, p<0.001, +46%), when the number of calls was directly related to HI values (p < 0.01). ECG diagnoses of new-onset atrial fibrillation were significantly increased 24 h after days with an HI >/= 44 (12 +/- 7 versus 8 +/- 3, p<0.01, +50%). ECG diagnoses of ST-elevation acute myocardial infarction, in contrast, remained substantially unchanged. No significant gender or age (>70 versus <70 years) differences were observed (chi-squared p not significant); increased rates of EMS callings were found 48 h after days with an HI >/= 44 in hypertensive patients (131 +/- 42 versus 78 +/- 26, p<0.001, +68%) and subjects with prior cardiovascular disease (137 +/- 43 versus 89 +/- 22, p<0.001, +54%). CONCLUSIONS: Increased work burden for EMS assessed with prehospital telecardiology screening accompanies heat waves because of subjects calling for suspected acute heart disease. Prehospital screening with telecardiology support may be of help in identifying subjects who do not require hospitalization in the event of heat waves with increased calls to EMS.