To conduct a cross-cultural adaptation of the Bath Ankylosing Spondylitis Functional Index (BASFI) into Brazilian-Portuguese language and to assess its measurement properties.The BASFI was translated by four rheumatologists and three English teachers. The translated questionnaire was applied to ankylosing spondylitis patients by trained observers, and self-administered in three moments: days 1, 2, and 14. The validity was assessed analyzing the association of BASFI and functional capacity measures (cervical rotation, intermalleolar distance, Schober's test and occiput-to-wall distance). The internal consistence was tested by Cronbach's α coefficient and the reliability by test-retest (intraclass correlation coefficient - ICC).A total of 60 patients with ankylosing spondylitis was included: 85% male, mean age 47 ± 12 years, and mean disease duration 20 ± 11 years. The intra-observer test-retest (two-week interval) reliability showed a high ICC (0.999, 95% CI: 0.997-0.999) and a high internal consistency (Cronbach's α coefficient: 0.86, CI 95%: 0.80-0.90). Considering the validity, the BASFI indices were correlated with cervical rotation (0.53, P < 0.001) and with intermalleolar distance (0.50, P < 0.001).The BASFI Brazilian-Portuguese version is reliable and valid for assessment of patients with ankylosing spondylitis.