['Medical Priority Dispatch System (MPDS) protocols are used to determine the appropriate level of emergency medical services (EMS) response that is sent to care for patients in the prehospital setting. The objective of this study was to determine the proportion of patients with abdominal pain who would benefit from advanced life support (ALS) when called for by these protocols.', 'All 9-1-1 calls were processed using MPDS protocols to determine whether the patient required ALS or basic life support (BLS) services. Consecutive patients having an ALS response for a chief complaint of abdominal pain were included. Dispatch decisions that did not follow the MPDS protocols, and cases taken to facilities other than the primary study hospitals, were excluded. EMS run sheets and hospital records were reviewed to determine: 1) whether prehospital ALS interventions were required, 2) emergency department (ED) disposition, 3) hospital course, and 4) final diagnosis. Calls were classified according to the need for ALS and the seriousness of the subsequent diagnosis. Data analysis was performed by determining 95%, confidence intervals (CIs).', 'Of the 343 patients classified as 1C1 or 1C2 who were transported by ALS during the time period, 227 (67%) were transported to the study hospitals. Nine (4%) were excluded because of inappropriate dispatch, leaving 218 for analysis. Hospital records were available for 186 (86%) cases, of which 12 (6%; CI 3%, 9%) were potentially life-threatening, requiring ALS intervention. Seventeen (9%; CI 5%, 1%) were non-life-threatening, but potentially benefited from ALS intervention. The remaining 157 (84%; CI 79%, 89%) were classified as not requiring ALS.'] Do effectiveness of a medical priority dispatch protocol for abdominal pain?