A study was conducted to evaluate the effect of characteristics patients' gender, age, weight, height, and body surface area, as well as the concurrent or recent use of opioids, ethanol and tobacco, on opioid dose requirements during administration of patient-controlled analgesia (PCA). Data were collected retrospectively from the medical records of 150 patients who underwent open cholecystectomies during an 18 month period at one institution. Demonstrable inter-patient variability in patterns of PCA use was observed. The results of the study demonstrate that during the first 48 hours of PCA therapy, patient age, height, weight, body surface area, gender, smoking, alcohol use, and preoperative opioid use may have significant influence on opioid analgesic use (p < 0.05). The data support the hypothesis that patient-specific factors may contribute to the variability observed in patients' PCA analgesic dose requirements, and these factors should be considered when selecting a proper demand (bolus) dose for PCA therapy.