The aim of this clinical study was to evaluate whether or not an association exists between the presence of enamel craze lines and the prevalence of tooth sensitivity (TS) after in-office bleaching. Subjects that met the inclusion criteria (N = 23) were screened to detect the existence of enamel craze lines. In total, 460 teeth were subjected to bleaching where 49 % of them presented enamel craze lines. After bleaching (15 % hydrogen per oxide), the subject were asked to rate the level of TS by answering a self-administered questionnaire. The majority of subjects (91 %)experienced TS at the first day of bleaching. The TS prevalence decreased gradually to 22 % at second day, to 17 % at third day, and to 9 % at fourth day. After the fourtf day, no subject reported TS. While 15 % of teeth with no craze lines also showed TS. A positive but weak correlation (r = 0.214) was found between the existence of enamel craze lines and TS. In this clinical study, higher incidence of TS was found with the use of 15 % hydrkedogen peroxide bleaching agent compared to the previous studies. Patients who would undergo in-office bleaching informed that tooth sensitivity is a very often side effect but it may disappear within 1 week.