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As each clinical procedure must be justified by a corresponding diagnosis in Japanese health insurance claim (HIC), unconfirmed diagnoses marked as rule-out diagnoses will be written on an HIC. However, little is known about the statistical profiles of such rule-out diagnoses.To illustrate the basic statistical profiles of rule-out diagnoses.We analysed all the diagnoses on 169 622 outpatient HICs in May 2006 from corporate health insurance societies. The proportions of additional diagnoses and rule-out diagnoses in each of the major disease categories defined by the Classification of Diseases for the use of Social Insurance, which is based on the International Statistical Classification of Diseases and Related Health Problems 10th Revision were calculated.There were 442 010 diagnoses on the HICs, of which 96 253 (21.8%) were principal diagnoses and 345 757 (78.2%) were additional diagnoses. Of the principal diagnoses, 1022 (1.1%) were rule-out diagnoses. Of the additional diagnoses, 19 308 (5.6%) were rule-out diagnoses. The percentage of rule-out diagnoses in the additional diagnosis category was significantly higher than that in the principal diagnosis category. Among the major disease categories, neoplasms showed the highest percentage of rule-out diagnoses for both principal diagnosis and additional diagnosis.The existence of rule-out diagnoses affects the results of statistics based on HIC data. Japanese statistics based on HIC data should be improved by utilizing the information on rule-out diagnoses.