The impact of the New Mexico safety belt law on patterns of injury, severity, and cost was evaluated (using an emergency department data base). Data collected from November 1985 through February 1986 compared the 2 months before the January 1986 law with 2 months after. Before the law, 22.2% of the 379 injuries included abrasions, contusions, and lacerations to face, neck, and head. Following the law, this category constituted only 13.8% of 356 injuries, representing a reduction of 38% (P less than .05). Cost and injury severity scores (ISS) for 436 injured patients covered by the law were not significantly different between the pre- and post-seatbelt law periods. However, significant cost and severity differences were observed after the law between belted and nonbelted occupants: +2,569 compared with +662; ISS of 3.6 compared with 2.0 (P less than .05). Methodologic problems of an emergency department-based study and the need for E coding (external causes of injury) are discussed.