Acute aortic dissection (AAD) cases are thought to have high blood pressure (BP) on admission; however, little data are available on BP prior to admission. The purpose of this study was to investigate systolic blood pressure (SBP) very early after symptom onset and before hospital transfer in patients with AAD to determine whether SBPs were high, and also whether SBPs were higher or lower compared with SBPs at hospital admission. We obtained results using three-year data derived from the Tokyo Acute Aortic Super Network Database. First, we selected 830 patients with AAD for which the “duration from symptom onset to first medical contact by ambulance crews” (SO-FMC) was within 60 min. We examined the SBPs of such patients. Next, we selected 222 patients with AAD whose SBPs were measured both at FMC, within 15 min after symptom onset, and at hospital admission, and compared SBPs at FMC with those at hospital admission. Among types A (n = 190) and B (n = 117), in patients with an SO-FMC ≤ 15 min, the median SBP was 100 mmHg and 178 mmHg (p < 0.001), respectively; 9% and 50% (p < 0.001) of such patients, respectively, exhibited an SBP ≥ 180 mmHg; and 43% and 10% (p < 0.001) of such patients, respectively, had an SBP < 90 mmHg. Of patients with types A (n = 124) and B (n = 98) AAD whose SBPs were measured both at FMC, within 15 min after symptom onset, and at hospital admission, SBPs at FMC were higher than those at hospital admission for the SBP ≥ 180 mmHg subgroups of both type A (194 mmHg vs. 159 mmHg, p < 0.001) and type B (199 mmHg vs. 186 mmHg, p < 0.001). Approximately 10 min after symptom onset and before hospital transfer, the measured SBPs of many patients with type A AAD were not necessarily high. However, the SBPs of cases with type B AAD were high as previously reported for SBP on admission. In addition, for the subgroup of SBP ≥ 180 mmHg at FMC within 15 min after symptom onset, SBPs at FMC were significantly higher than those at hospital admission for both types A and B; the higher SBP at symptom onset may have been partially associated with being a trigger of AD.