In August 2003, France sustained an exceptional heat wave. Heat-generated pathologies (dehydratation, heat stroke, cardio-vascular diseases) were responsible for additional biological analysis orders at the Saint-Antoine Hospital biochemistry laboratory in Paris from 4 to 18 august, compared to the same period in 2002. Variations were: + 17.6% for analysis orders, + 30.1% for ionograms, + 28.9% for plasma troponins I and + 58.6% for blood gazes analysis. Women and patients older than 75 years ratios were higher in august 2003. Biochemistry results analysis showed higher frequency of elevated plasma sodium, creatinine and troponin in 2003, confirming that most of patients admitted during heat wave were affected by heat-related diseases. Finally, laboratory excess activity was performed and quality was maintained, in spite of reduced staff and unusual climatic conditions.