There are twenty fields (fourteen before April 2007 and seven before April 2002), diag_01 to diag_20, which contain information about a patient's illness or condition. The field diag_01 contains the primary diagnosis. The other fields contain secondary/subsidiary diagnoses.

The codes are defined in the International Statistical Classification of Diseases, Injuries and Causes of Death. HES records currently use the tenth revision (ICD-10). Prior to April 1995, the ninth revision was used (ICD-9).

Diagnosis codes start with a letter and are followed by two or three digits. The third digit identifies variations on a main diagnosis code containing two digits. The third digit is preceded by a full stop in ICD-10, but this is not stored in the field.

annnna = A valid ICD-9 or ICD-10 diagnosis code

annnnn = A valid ICD-9 or ICD-10 diagnosis code

null = not applicable