We performed detailed DNA sequencing analysis on type III collagen cDNA from 58 patients with either intracranial artery aneurysms or cervical artery dissections. The 58 patients were of seven different nationalities; among the patients were three pairs of relatives, so that 55 were unrelated, and of these, 29 had at least one blood relative with either an intracranial artery aneurysm or a cervical artery dissection. The age of the patients at the time of diagnosis ranged from 15 to 68 years (mean ± SD = 40.3 ± 11.0). The study group consisted of 25 males and 33 females. The analysis covered 3,232 nucleotides of significant (nonredundant) sequences per allele; therefore, we analyzed as many as 355,520 nucleotides. Mutations in the coding sequences for the triple-helical domain of type III collagen were excluded in 40 individuals with intracranial aneurysms and 18 individuals with cervical artery dissections. Direct sequencing of polymerase chain reaction products allowed mutations to be excluded with a high degree of confidence. Mutations that markedly decreased expression from one allele were also excluded in 42 of the 58 individuals, since the presence of both bases at one or more polymorphic sites in the 42 patients showed that two alleles were transcribed. The results indicated that mutations in the gene for type III procollagen (COL3A1) are not a common cause of either intracranial artery aneurysms or cervical artery dissections.