On 20 January 1990, twenty-three Tursiops truncatus (Common Bottlenose Dolphin) carcasses were found scattered around the interior shoreline of East Matagorda Bay, TX. Few accounts exist to document the presence of live or dead Common Bottlenose Dolphins inside the boundaries of East Matagorda Bay before or after the die-off. We conducted a review of areal data for East Matagorda Bay and the original investigation of the January 1990 mass die-off. Information we examined included the history of natural and anthropogenic changes to the area, dolphin stranding records, small-boat visual surveys, and dolphin dorsal-fin photographic identification. Natural events preceding the discovery of the dolphin carcasses were likely factors in the mortalities; however, the timing of engineering projects that modified access points between the bay and the Gulf of Mexico may be an additional factor that contributed to the cause of this unusual mortality event.