Mental illnesses are common, serious, brain disorders that seriously hamper an individual's ability to think feel and act.Studies have shown that risk of developing an illness is increased if another family member is similarly affected, suggesting strong hereditary component. (N.I.M.H.Report, 1997.)

Study sample consists of 721 mentally ill patients.Patients were diagnosed in the Outpatients Department of Psychiatry, Pakistan Institute of Medical Sciences Islamabad. Dept. of Psychiatry was visited from June 1998 to September 1999 for collection of data.

Study comprises 721 mentally ill patients. Bipolar (manic-depressive) patients are 62.97% (n=454) Personality disorders 26.21% (n=189) and schizophrenics are 10.89% (n=78).Ratio of females to males is 100♀♀: 100.27♂♂.

Mean age at diagnosis of mental illness in sample is 25.86 ± 0.40 years. Majority of patients fell in age cohort ranging from 10 years to 29 years. (n=477:66.16%).

The study shows total number of inbred marriages to be 400(55.47%) while outbred marriages are 321(44.52%). The coefficient of inbreeding (F) for mental illness patients is 0.0348 and that of control sample is F=0.0273.

There were 220 (30.51%) patients with positive family history 29(4.02%) with negative family history and 472 (54.66%) where occurrence of disease sporadic. Consanguinity is more in positive(23.35%) and negative group(3,30%) while in sporadic cases outbred cases are more in number(39.43%).

This study shows consanguinity as an important factor in the onset of mental illness and is a fertile base for further molecular genetic studies to locate genes contributing in the development of illness