Most Parkinson's disease (PD) patients are suspected to have idiopathic Parkinson's disease which is occurred for no specific known causes. Scientists do not yet know accurately what causes PD to develop in some people, while not in others. But they have recognized age as an important factor that play crucial role in PD development in some individuals. For example, people below age 50 usually bear low risk of developing PD among them, while people over age 50 bear high risk of developing PD among them.

Although some experts identified some families, who are sharing a gene that is helpful to PD development, this is not so much significant relative to the total victim population of PD. Most cases of PD shows irregularity, i.e. genetics factors have not been found to be play a clear and apparent role in the PD development. Scientists think that for most cases, the cause of PD is perhaps a combination of several genetic and environmental factors and these factors varies from person to person.

Genetic causes/risk factors: Researchers say that only five percent patients develop Parkinson's disease due to genetic reasons - not high cholesterol or smoking. This doesn't mean however that those who have no family history of PD has no chance of developing the disease. With a family history of PD, the chances for one to develop it is slightly higher than those who have no family link. Researchers have identified 13 separate genes that are connected with PD development in human body. Recent evidence also shows that a common gene defect may also help in the development of both Parkinson's and Alzheimer's disease. Although genetic factors play a very small role in PD existence and progress, they are currently taken into great consideration in research because genetic factors can help understand the rare genetic forms of PD, thus helping greatly in the identification of its many forms.

Environmental causes/risk factors: Parkinson's disease may be also caused by various environmental factors such as toxin, but study shows that any environmental factor can not be considered alone as responsible for causing the disease. Some epidemiological research has recognized several environmental factors such as rural living, exposure to pesticides etc. that may be associated with PD development.

Previous Medical History. A recent study shows that those people, who have a head injury in the past, are more likely to develop PD than those who have no medical history of head injury. Aside from that, it is believed that some diseases which affects the brain like encephalopathy, cerebrovascular accidents (stroke) and arteriosclerosis (a disease in which arteries, especially those leading to the brain hardens), can cause Parkinson's Disease in the later part of life.

Drugs. The use of certain drugs also increases the chance for one to develop parkinsons disease. Drugs which are specifically psychoactive are examples of these PD causing drugs. Antipsycholtics like Haloperidol. Thorazine and the like are used in patients which are psychotic in the attempt to block dopamine receptors in the brain. Thus, high dosage or uncontrolled use of these drugs can lead to PD.