Transcranial Doppler (TCD) is a technique that evaluates blood flow velocity in intracranial vessels. It uses a 2-MHz probe and a Doppler signal analyzer. Absence of an acoustic window is a considerable problem for clinical utilization of TCD because cerebrovascular patients are frequently elderly. Previous reports suggest a higher prevalence of inadequate temporal acoustic window (TAW) in aged subjects and in females. A consecutive series of 624 subjects (376 males and 248 females, age range 2-86 y) were evaluated by standard TCD examination, to assess the contemporary absence of any signal corresponding to insonated basal arteries, defined as inadequate acoustic window. The rate of inadequate TAW was 8.2%, that of inadequate occipital acoustic window (OAW) was 9.0%. Prevalence of inadequate TAW was higher in females than in males, and OAW was higher in males than in females. Influence of aging on the presence of inadequate acoustic window is confirmed for temporal, but not for the occipital window. Different anatomical characteristics of the 2 regions could explain the different prevalence of TAW and OAW.