Gatherings of large numbers of people at concerts, sporting events, and other occasions lead to an assembled population with a potential for a wide variety of illnesses and injuries. The collection of large numbers of people in a single location has led some authors to recommend the placement of resuscitation equipment or other medical services in close proximity to these activities. These recommendations not withstanding, data on the frequency of critical illness at mass gatherings (a group exceeding 1000 persons) are difficult to ascertain. Therefore, it was the purpose of this study to describe the incidence of critical illnesses among assembled populations at mass gatherings. An observational prospective study was conducted involving patient encounters at a large, multipurpose, indoor mass-gathering complex in Houston, Texas occurring between Septemver 1, 1996 and June 30, 1997. Demographic, treatment, disposition and diagnostic data were analyzed in a computerized database. Of the 3.3 million attendants to the 253 events analyzed during the 10-month study period, there were 2762 (0.08%) patient encounters. Fifty-two percent were women. Mean age was 32 +/- 15.6 years. Of these patients, 51.1% were patrons and the remaining patients were employees or contractors of the facility. A wide variety of illness was seen with trauma (39.5%), headache (31%), and other medical complaints (29.5%) being most frequent. Disposition of the patients included 95.3% being discharged to go back to the event and 2.2% being counseled to seek other medical attention. One hundred twenty-nine patients (4.7%) were referred to the Emergency Department (ED); of these, 70 were transferred for abrasions, lacerations, or skeletal injuries and 13 for chest pain. Of those referred to the ED, 50 (38.7%) patients were transported by ambulance and only 17.4% were admitted to telemetry, with none admitted to an ICU. It is concluded that critical illness at mass gatherings is infrequent, as seen in this study, with very few being admitted to telemetry and none to an ICU. Careful consideration of cost-benefit should occur when determining allocation of resources for these activities.