The number of positron emission tomography (PET) facilities has rapidly increased in Japan in recent years. We assume there has been a downward spiral that low reimbursement prices for PET scans by public health insurance have driven these facilities to provide cancer screening, leading to a decline in efficiency with more cancer screening. The purpose of this study was to confirm this decline and clarify the determinants of efficiency.Questionnaire survey data from 65 facilities that provided PET cancer screening were used. Data envelopment analysis (DEA) was used to measure the efficiency of each facility. Multivariate regression analysis was then performed with each DEA score as a dependent variable, with other potential factors that might affect efficiency as independent variables.We found that from 2004 to 2006 efficiency in PET facilities declined. However, significantly greater efficiency was observed for those facilities that started cancer screening earlier, were located in an area with an aging population, and were clinics and general hospitals. Conversely, significantly lower efficiency was observed for facilities with higher market share and price for PET cancer screening.Our findings on decreasing efficiency imply that reimbursement prices should be raised to halt the downward spiral.