The authors investigated whether neutrophil to lymphocyte ratio (N/L) can contribute to the diagnosis and risk assessment in patients with type A acute aortic dissection (AAD). The authors studied 120 consecutive patients with type A AAD (group I) and compared them with 121 consecutive patients with chronic aneurysms (group II) and 121 age- and sex-matched healthy subjects (group III). It was found that white blood cell count, N/L, D-dimer and C-reactive protein were significantly higher in group I versus both groups II and III (p < 0.001 for all comparisons). White blood cell count and D-dimer were much higher in patients who died compared to survivors in group I (p = 0.023 and p = 0.033, respectively). A cutoff value of N/L >4.6 was associated with 0.89 sensitivity and 0.91 specificity for AAD. High N/L may contribute to the diagnostic evaluation and prompt immediate therapy in patients with type A AAD.