We hypothesised that ethnicity may influence the circadian pattern in acute myocardial infarction (MI), in view of the potential differences in genetic background, cardiovascular risk factors and cultural habits. To test our hypothesis, we studied 340 consecutive acute MI patients (268 males; mean age 61.6+/-12.3 years) from two different city-centre teaching hospitals in Birmingham (United Kingdom) and Alicante (Spain). A different circadian rhythm in MI onset was observed between the ethnic groups (p=0.001), with a significantly higher number of acute MI onset occurring between midnight and noon in British Caucasians and Indo-Asians. In contrast, Mediterranean Caucasians showed the converse circadian pattern, with most of the acute MI events happened between noon and midnight. Indo-Asian patients were the youngest patient group and showed the highest prevalence of diabetes and increased body mass index. Mediterranean patients had the highest prevalence of smokers but their mean serum cholesterol was the lowest. No differences in sex, blood pressure, height and weight were observed. In conclusion, this study has shown a different circadian rhythm in acute MI onset between 3 ethnic groups from two different city-centre teaching hospitals in Birmingham (United Kingdom) and Alicante (Spain) and, for the first time, provide data in the Indo-Asian population. Further studies are required to determine the pathophysiological mechanism(s) underlying these differences.