OBJECTIVES: Parkinson’s disease has the second largest number of patients among 56 designated diseases/syndrome of the Specified Disease Treatment Research Programme for rare and intractable disease programme. The purpose of this study were to investigate prescribing patterns of Parkinson's disease and its pharmaceutical expenditures. METHODS: We analysed the Social Health Insurance (SHI) claims data processed from February to April 2011 provided by the Planning, Review and Research Institute for Social Insurance and Medical Program.  RESULTS: During the three months, 52,851 patients in Social Health Insurance programme received medical treatments that cost 9,391,451,520 JPY. Average costs for inpatient care without meal expense and for outpatient care were 263,782 JPY and 33,209 JPY respectively. Except the cases that fell into Diagnosis Procedure Combination (DPC), 37% of the total cost, 3,515,091,420 JPY was for pharmaceutical expenditure including dispensing fee. Only 7,437,873 (23.5%) of 31,699,153 dispensed drugs were generics on a volume basis. If all the pharmaceutical that have generics had been substituted, estimated 899,586,280 JPY would have been saved in the study period by simple arithmetic. Generic usage is fewer in the elderly than in the younger generation in all therapeutic categories. CONCLUSIONS: Percentage of pharmaceutical expenditure in total health expenditure for Parkinson’s disease is high. Our study find that generic substitution rate is low in general and even lower in the elderly. This might be partly related to the medical expense subsidy under the Specified Disease Treatment Research Programme, which exempts eligible patients from co-payment according to their income. There seems to be a room for prescribing pattern change for cost optimization and further study is expected.