A pair of 17-year-old male, monozygotic twins concordant for obsessive illness is reported. Clinical evaluation of these twins is impressive in that their clinical course has been strikingly parallel. Both twins began to manifest compulsive mannerisms at about age 2½ and have continued to have some daily symptoms to the present time. Both entered a severe phase of illness in early adolescence; though both manifested all the characteristics of obsessional illness. Twin A was primarily occupied with ritual and Twin B with ruminative, anacastic, phobic thoughts. In each there was a marked diminution of symptoms at about age 14 following medical and psychiatric attention.Their mother has a mild unclassified neurotic illness without obsessive symptoms. The twin's father has a history of an erratic youth and there is a history of marked obsessive traits. The paternal grandmother manifested phobic and obsessive symptoms.Family and genetic studies from the literature on obsessive illness are reviewed.