The frequency with which conversion reaction diagnoses are inappropriately applied to persons with physical diseases was estimated in a sample of medical hospital patients diagnosed hysteric. This was accomplished by ratings based on file information available at the time of diagnosis and at a 10-year follow"up, and a count of patients whose "conversion" symptoms were later diagnosed as physical illnesses. The estimated false positive rate was 25%. The symptoms most characteristic of the false positives were degenerative diseases and structural failures affecting the spinal cord, peripheral nerves, bones, muscles, and connective tissues. An effort to identify psychological parameters capable of separating true conversion reactions from false positives with a variety of behavioral symptoms and Minnesota Multiphasic Personality Inventory scores was not productive.