BACKGROUND: It is well known that induction with large dose of propofol causes high incidence of apnea and marked decrease in mean arterial blood pressure. So, it is wise to titrate adequate dose to induce sleep for induction of anesthesia. In this study, we evaluated the incidence and duration of apnea and the frequency of unconciousness according to variable single dosage of propofol. METHOD: We evaluated 99 adult patients (ASA class 1 or 2) who was undertaken general anesthesia. We allocated patients randomly into three groups according to dosage of propofol. Without premedication, we administered propofol 1 mg/kg(group 1) or 1.5 mg/kg(group 2) or 2 mg/kg(group 3) over 10 seconds as an indution agent. To assess apnea, we observed chestwall movement and ETCO2 waveform. Unconsciousness was assessed by asking the patient to open his/her eyes. RESULTS: As the dose of propofol increased, the frequency and duration of apnea significantly increased. The frequencies of apnea were 44% in group 1, 63% in group 2 and 83% in group 3. The durations of apnea were 27 12sec, 60 33sec, 74 34sec respectively. And the frequencies of unconsciousness were 66% in group 1, 75% in group 2 and 97% in group 3. CONCLUSION: Incidence and duration of apnea and frequency of unconciousness increase as injection dose of propofol increase. To get unconciousness in adult patients, adequate single-dosage of propofol is more than 2 mg/kg of propofol.