We compared 1998 hospitalizations in active-duty US military personnel for possible temporal association with anthrax immunization. Immunization, demographic, and hospitalization data were analyzed using Cox proportional hazards modeling for hospitalization within 42 days of vaccination. Discharge diagnoses were aggregated into 14 International Classification of Disease, Ninth Revision, Clinical Modification (ICD-9-CM) categories. Approximately 11% of subjects received one or more doses of vaccine during 1998; those immunized were more likely to be younger and male. Lower hospitalization rates were observed across doses and diagnostic categories among the immunized. Adjusted risk ratios for hospitalization by diagnostic category suggest that immunized service members were at equal or lesser risk for hospitalization than the non-immunized.