Because estrogen has numerous effects on dopamine neurotransmission, many researchers are interested in its possible use to either slow the progression or reduce the risk of Parkinson's disease (PD). The incidence of PD is greater in men than in women. Gender differences in neurotoxicity have been observed, and basic research in experimental animals indicates that estrogen protects neurons from various forms of injury. However, the results of retrospective surveys of the neuroprotective effects of estrogen replacement in PD have been mixed, with some showing no effect on risk and others showing a reduction in risk. A mildly significant gender difference in disability and quality-of-life reporting has been noted, with women citing greater disability and reduced quality of life. Gender differences have been shown in response to treatment of PD, for example, in how levodopa is metabolized--women have greater levodopa bioavailability. In the Parkinson's Disease on Estrogen Therapy Replacement in the Menopause Years (POETRY) study, participants were found to have improved scores on the Unified Parkinson Disease Rating Scale. Based on the POETRY results, it is hypothesized that estrogen replacement therapy (ERT) may lead to improvement in PD symptoms and provide an opportunity to reduce the dosage of antiparkinsonian medication in women.