The growth and rupture of 40 cerebral aneurysms was studied in 36 patients (14 men, 22 women; were average age, 51.8 years). Aneurysms were classified into five types according to the intraoperative findings: type 1, uniformly thin, smooth surface; type 2, thin neck and thick wall, smooth surface with or without red and/or transparent portions; type 3, uniformly thick wall, smooth surface with or without red portions; type 4, thick neck, bubbled or loculated thin wall at dome with or without red and/or transparent portions; type 5, thick wall in entirety, irregular surface with or without red portions. Five were type 1, six type 2, and 12 type 3. In four of the type 2 aneurysms, turbulence could be seen at the neck. In seven of the type 3 aneurysms, red and/or transparent portions were observed in the wall. Thirteen were type 4; nine of which had a bubbled or loculated wall with or without red and/or transparent portions. Four were type 5, with scattered red portions but a thick wall. Type 1 aneurysms were 2-5 mm, most of types 2 and 3 were 3-6 mm, type 4 were 3-13 mm, and type 5 were more than 9 mm. Types 1 and 2 had few local changes in the wall, suggesting that aneurysms at this stage are stable. Type 3 is considered to be a transitional stage to type 4 from type 2. Type 4 aneurysms had some local changes within the wall including bubbles or loculi. We concluded that aneurysms exceeding 4 mm have local pathologic changes in the wall and are critical.(ABSTRACT TRUNCATED AT 250 WORDS)