Chewing fresh leaves of the khat plant (Catha edulis), represents a widespread habit with a deep-rooted socio-cultural tradition in Yemen. Khat is chewed for its central stimulant properties and to dispel feelings of fatigue, and its use is believed to be associated with disturbance of mood. Yemeni researchers studied the effect of chewing Khat leaves on human mood. A prospective study was conducted in the Faculty of Medicine and Health Sciences, University of Sana’a, Yemen. It comprised 200 healthy volunteers, interviewed on two occasions 1 week apart. Subjects either chewed khat at least 3 h daily for 3 days or abstained from chewing for at least 7 days prior to mood assessment using the Hospital Anxiety and Depression scale. Subjects were studied in random order. Using the Hospital Anxiety and Depression scale, there was a significant increase in median score on the scale, indicating mood disturbance in the khat arm of the study compared with the control arm. The effect was particularly evident shortly after use of khat.