This prospective study was undertaken to determine the incidence and factors predisposing to vaso-vagal reactions during venous cannulation in an ambulatory surgery population. In 141 ambulatory surgery patients, signs and symptoms of a reaction together with mean arterial pressure and heart rate were recorded at 1-min intervals during and for 6 min after venous cannulation. Overall, 10.6% of patients were symptomatic (95% confidence interval [CI] 6%-17%). The incidence was 16.6% (95% CI 8.4%-24.9%) in patients < or = 40 yr and 33.3% (95% CI 6.7%-60.0%) with a prior fainting history. Young age, duration or number of attempts at venous cannulation, and fainting history were independently associated with increased risk of a reaction (P < 0.03-0.004 by multiple repression analysis). Minimum mean arterial pressure was less in symptomatic patients than in those who were asymptomatic (58 mm Hg +/- 11.3 SD versus 82 mm Hg +/- 14.3 SD, P < 0.0001). We conclude that reactions occur commonly, particularly in the young or in patients with a history of fainting. Reactions are typically associated with significant hypotension that may require treatment.