Research findings have shown that birth order and season of birth are related to some of the mental disorders such as schizophrenia, mood disorders. autism, attention deficit disorder with hyperactivity and substance abuse, and these disorders are related to dopamine level. A retrospective study was designed to investigate 130 substance abusers (119 males and 11 females) which were treated during 1997 by the author. They administered a questionnaire inquiring about subjects' demographic characteristics, birth order and family history of substance abuse. Data were analyzed using descriptive statistical methods. Results showed that the frequencies of substance abuse in first born children were 33 (27%) and in the last children were 22 (19.5%), which are 1.7 times higher than expected rate and 1.9 times higher than the rates for middle children. Thus, the first borns, especially the first born sons, are at higher risk for substance abuse than other children. In terms of the season of birth, 58% of all of the subjects were born in the cold months of the year. Thus it seems that, like schizophrenia, substance abuse is related to the season of birth and 02 receptors. Also the risk of addiction for subjects born in December, January and February that production of melatonin is at the highest rate in this period, 1.5 times more than June, July and August