Summary:When we treat a patient with progressing stroke, STA-MCA bypasses in the acute stage may be effective to minimize the lesion size. However, there was no evidence to support this hypothesis. The aim in this study is to investigate the potential risk and safety of acute stage bypass surgery to determine whether this treatment would be reasonable or not.Over a 4-year period, from April 1 2002 to August 1 2006, we performed 55 STA-MCA bypasses, 23 in the acute stage and 32 in the chronic stage. The chronic stage patients were based on the selection standards of JET study (Japanese EC-IC bypass Trial). Each case was examined by IMP or Xe SPECT within 2 weeks of bypass surgery. Where hyperperfusion was evident, cases were divided into one of 2 groups, according to the location of the hyperperfusion: Group A demonstrating it locally, and Group B demonstrating it hemispherically. Symptomatic cases were also divided into 2 groups according to the degree of severity, with the mild group exhibiting such symptoms as headaches or transient psychological disturbance, and the severe group reporting convulsions or conscious disturbance, etc. We examined the frequency of hyperperfusion syndrome and the safety of STA-MCA bypasses performed in the acute stages.Of the 23 acute stage bypasses, 7 cases (30.4%) were classified as Group A and 2 cases (8.7%) as Group B, affecting 9/23 cases, or 39.1% of the acute total. Of these, there were 3 mild symptomatic cases (13.0%), and 1 severe case (4.3%). Of the 32 chronic stage bypasses, 4 cases (12.5%) were classified as Group A and 1 case (3.1%) as Group B, affecting 5/32 cases, or 15.6% of the chronic total.Of these, there was 1 mild symptomatic case (3.1%), and 2 severe cases (6.3%). There were no hemorrhagic transformations. According to these results, acute stage bypasses did not differ significantly from chronic ones with regard to the occurrence of symptomatic cases. (p 0.435; Fisher exact method)