The purpose of this study was to clarify the activities that trigger the onset of acute aortic dissection (AAD) and their relation to the occurrence of AAD. The study group comprised 444 consecutive patients referred for spontaneous AAD. From the hospital medical records, the activities that triggered AAD could be identified in 307 of these: most (86.6%) AAD episodes occurred in relation to physical (73.6%) or mental (13%) activities. In the older (≥61 years) population, AAD occurred significantly more frequent during sleep or rest than in the younger (≤60 years) population (16.9% vs 7.6%, p=0.020). The relationship between the time of onset and the triggering activity of AAD could be assessed in 267 patients. The onset of AAD was predominantly during the day: 63.3% of the episodes occurred between 06.00 h and 18.00 h, and were significantly more related to physical or mental activities than the nighttime events (95.3% vs 70.4%, p<0.0001). Most (86.6%) of the AAD episodes were related to physical or mental stress, particularly those that occurred during the day. (Circ J 2002; 66: 232 - 235)