C91

C91 may refer to :
 * Ruy Lopez chess openings ECO code
 * Lymphoid leukemia ICD-10 code
 * Brun C91, a 1991 sports prototype racing car built for Group C
 * Dowagiac Municipal Airport in Dowagiac, Michigan FAA LID
 * Paid Vacations (Seafarers) Convention (Revised), 1949 (shelved) code
 * C91FM, a former branding name for Christchurch New Zealand radio C93FM
 * Caldwell 91 (NGC 3532), an open cluster in the constellation Carina