The research objective is segmenting outpatients for CRM(Customer Relationship Management) in medical service. Using modified RFM(Recently, Frequency, Monetary) method based on frequency and profitability in the hospital, the data were analyzed with the data mining technique. The result can be summarized as follows : The outpatients were semented into the four groups: 1) the loyal patient group, who have kept visiting until recently and give high profitability; 2) potential loyal patient group, who give lower profitability but high frequency of use, 3) potential withdrawer patient group, who have lower frequency of use but give high profitability and; 4) withdrawer patient group, who give low frequency of use and have not visited recently.