Many studies have shown a relationship between obstructive sleep apnea (OSA) and accidents, but to our knowledge, none have investigated prospectively the effects of treatment with nasal continuous positive airway pressure (CPAP). CPAP was proposed to 973 patients, of whom 893 patients actually underwent CPAP. These patients were consecutively invited to enter a prospective follow-up study including a questionnaire before treatment and after 6 and 12 months of treatment; 547 patients completed the study (153 left the study, and only partial data were available for 193). The baseline questionnaire included questions concerning accidents in the previous 12 months, asking whether patients had had an accident and, if so, whether they felt that the accident(s) were related to sleepiness, and whether the patients felt that they had had near-miss accidents due to sleepiness. The questionnaires at 6 and 12 months included the same questions referring to the previous 6 months; the accidents reported on each follow-up questionnaire were cumulated and compared with the accidents during the 1-year period before treatment. The number of patients having an accident decreased with treatment for real accidents (from 60 to 36; p<0.01), as well as for near-miss accidents (from 151 to 32; p<0.01). The average number of accidents per patient also decreased, for real accidents (from 1.6±1.3 to 1.1±0.3; p<0.01) and for near-miss accidents (from 4.5±6.5 to 1.8±1.4; p<0.01). The cost, in terms of days in hospital related to accidents, decreased from 885 to 84 days. With caution due to the absence of a control group, it is suggested that treatment with CPAP decreases the number of accidents occurring in OSA patients. This result may have important implications in the evaluation of the cost/benefit ratio when treating OSA patients.