["To study the characteristics of P300 in Tourette's syndrome (TS) with and without attention deficiency and hyperactivity disorder (ADHD).", 'Auditory evoked P300 were recorded in 19 TS only (TS-ADHD) children, 15 TS with ADHD (TS + ADHD) children and 20 unaffected control subjects, and their waveforms, amplitudes, latencies and topographies were compared at Fz, Cz, C3, C4 and Pz.', 'The TS + ADHD group showed shorter latencies than control subjects at all electrode sites (P<0.05 or 0.01), and the TS-ADHD group at CZ and PZ (P<0.05); however, there was no significant difference between control subjects and the TS-ADHD group. The TS-ADHD group showed smaller amplitudes than the control group at all electrode sites (P<0.05), and the TS + ADHD group at Cz (P<0.05); however, there were no significant differences between control subjects and the TS + ADHD group. There was no significant difference in the prevalence of abnormal waveforms between the control, TS, TS-ADHD and TS + ADHD groups, but there were significant differences in the variability of localization of P300 between the control and the TS group (P=0.003), control and TS + ADHD groups (P=0.000), and the TS-ADHD and TS + ADHD groups (P=0.039). P300 in the TS + ADHD group tended to spread out to the left and that of the TS-ADHD group tended to spread out to the right.'] Do p300 differences exist between Tourette 's syndrome with and without attention deficiency and hyperactivity disorder in children?