Sinus pericranii is an uncommon congenital cranial venous malformation that may become symptomatic in the pediatric population. Both dominant and accessory sinus pericranii, as determined by the intracranial venous drainage pattern, have been described. The dominant variety drain a significant proportion of the intracranial venous outflow while the accessory variety have minimal or no role in this. Classic teachings hold that dominant sinus pericranii should never be treated while accessory sinus pericranii may be safely obliterated. This determination of dominance is solely based on a qualitative assessment of standard venous phase catheter cerebral angiography, leaving some doubt regarding the actual safety of obliteration. In this paper the authors describe a simple and unique method for determining whether intracranial venous outflow may be compromised by sinus pericranii treatment. This involves performing catheter angiography while the lesion is temporarily obliterated by external compression. Analysis of intracranial venous outflow in this setting allows visualization of angiographic changes that will occur once the sinus pericranii is permanently obliterated. Thus, the safety of surgical intervention can be more fully appraised using this technique.