An anxiety state is a cluster of symptoms based on fear, the source of which is not recognized by the patient. The anxiety may be sustained, but more often is episodic from a few minutes to hours or days. The chief symptoms are those of anxiety and its physical concomitants in the absence of other illness and independent of specific situations; they include breathing and swallowing difficulties, palpitations, dizziness, irritability, and faintness. Depression, phobias, and obsessions are common complications. The syndrome occurs in young adults of both sexes and its course may be acute or chronic. Acute forms may present in epidemics which are culture bound, e.g., Koro and epidemic hysteria. Chronic forms can fluctuate and persist for many years without other physical or psychiatric syndromes appearing. Between attacks the patient feels fairly well. Prevalence is estimated at 2 to 5 per cent of the normal population and 6 to 27 per cent of psychiatric outpatients. The syndrome requires differentiation from depressive and phobic disorders and from physical conditions which produce anxiety.