Pathological laughing and crying (PLC) is an emotional expression disorder in which patients experience relatively uncontrollable episodes of laughing or crying, or both. The laughter or crying are unconnected to triggering stimuli which would have led to laughing or crying before the onset of the condition. PLC is therefore quite different from mood disorders, where laughing and crying are expressed in tandem with feelings of happiness or sadness. PLC is a frequent consequence of brain injury, seen in approximately 10-20% of stroke, 40% of Alzheimer’s, 7-10% of multiple sclerosis, 20-50% of amyotrophic lateral sclerosis, 5-10% of traumatic brain injury patients, 6% of Parkinson's patients as well as about 5% of noncerebellar type multiple system atrophy (MSA), and 37% of patients with the cerebellar type of MSA .

Studies have reported an inconsistent pattern of association between PLC and major depression, sometimes an association is found, other studies find none.

PLC has long been thought to result from loss of voluntary inhibition of a postulated centre for laughing and crying in the upper brainstem . More recent authors have suggested that prefrontal cortex is involved or that lesions in the cerebro-ponto-cerebellar pathways influence cerebellar structures that adjust the expression of laughter and crying

Patients administered tricyclic antidepressants show significant improvements PLC, with no effect on coexisting depression Reports that patients show partial to full remission within a week of starting low doses of SSRI suggest that serotonergic systems are involved. By comparison, major depression does not respond to SSRIs before three to five weeks of treatment. AVP-923, a compound consisting dextromethorphan and quinidine, has also shown significant theraputic effect.

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