Mutation of the class I beta-tubulin gene has been reported to be one of the mechanisms that cause resistance to paclitaxel. To assess the relationship between paclitaxel-resistance and class I beta-tubulin gene mutation in breast cancer, Japanese patients with breast cancer were screened for the class I beta-tubulin gene mutation. Total RNA was isolated from 82 breast cancer specimens and the corresponding normal tissues. Twenty-four of the 82 patients were treated with paclitaxel preoperatively and 12 of them did not respond to the treatment. Of the 82 breast cancer patients, 15 (18.3%) had silent polymorphism in exon 4, Leu217Leu (CTG/CTA). However, no mutations showing amino acid substitution of the beta-tubulin gene were detected in any of the patients, including 12 patients who did not respond to paclitaxel. Class I beta-tubulin gene mutation with amino acid substitution was not detected in 82 breast cancer specimens. Our results suggest that mutation of the class I beta-tubulin gene is unlikely to play an important role in the mechanism of resistance to paclitaxel in breast cancer.