Forty patients (ASA status I-III) recovering from major orthopedic or gynecological operations were investigated to evaluate analgesic efficacy and threshold concentrations of tramadol and its main metabolite O-demethyltramadol (M1) in serum during the early postoperative period, using patient-controlled analgesia (PCA) by means of the Abbott Lifecare Infuser. Following an individualized intravenous loading dose of 97.5 +/- 42.3 mg (mean, SD), tramadol demand doses were 20 mg with a limit of 500 mg within 4 h; the lockout time was set to 5 min. The duration of PCA was 20.5 +/- 4.8 h. During this time 8.0 +/- 5.0 demands per patient were recorded, resulting in an average tramadol consumption of 257.5 +/- 102.8 mg (including loading dose). Analgesia was mostly judged good to excellent. Side effects were only of minor intensity and never gave rise to concern. There were no statistically significant differences between the types of surgery. Tramadol proved to be about 1/6 to 1/10 as potent an analgesic as morphine when both intensity and duration of effect were considered. Minimum effective tramadol serum concentration (MEC) varied greatly and could be best described by a log-normal distribution (range 20.2-986.3 ng/ml, median 287.7 ng/ml). Intraindividual MEC variability was lower than intersubject variability (38.2 vs 59.1%). Median M1 concentrations were 36.2 ng/ml.