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Objectives: To establish the point prevalence of pathological laughing and crying (PLC) in multiple sclerosis (MS). To define associated neurological, emotional, and cognitive correlates of PLC. Design: A consecutive sample of 152 patients with clinically or laboratory definite MS were screened for PLC, defined as sudden, involuntary displays of laughing or crying or both, without associated subjective feelings of depression or euphoria. Thereafter, a case-control design was followed with patients with PLC matched to patients with MS without PLC on age, gender, physical disability (Expanded Disability Status Scale), duration of MS, and premorbid IQ. Setting: An MS outpatient clinic, the population representative of a large urban catchment area. Patients: Fifteen of 152 patients had PLC, 11 of whom (mean [SD] age, 43.7 [8.3] years, 7 women) agreed to further testing. Thirteen patients with MS without PLC acted as controls. Main Outcome Measures: Neurological examination, Pathological Laughter and Crying Scale, Hospital Anxiety and Depression Scale, 28-item General Health Questionnaire, and the Wechsler Adult Intelligence Scale-Revised. Results: The point prevalence of PLC in MS was 10%. Patients had a mean Expanded Disability Status Scale score of 6.5, had had MS for a mean (SD) of 10 (5.8) years, and had entered a chronic-progressive phase of their illness. Pathological laughing and crying was not associated with disease exacerbations. Compared with controls, patients were not more depressed or anxious, but had a greater decline in IQ. Conclusions: Pathological laughing and crying as distinct from emotional lability affects 1 in 10 patients with MS. It occurs in severely physically disabled patients, generally with long-standing disease. The presence of cognitive deficits relative to controls implies more extensive brain involvement.