Knowledge of a patient's left mainstem bronchial diameter would provide a useful guide to choosing the appropriate double-lumen tube (DLT) size for that patient. Therefore, the authors investigated the possibility that left bronchial diameter could be predictably estimated from a patient's size and sex. The routine preoperative chest X-rays of 100 adult male and female patients of known age, height, and weight were examined. The left bronchial diameter was measured on the posteroanterior (PA) chest x-ray, and a formula was used to correct for the magnification of intrathoracic structures encountered on the PA view. Multiple linear regression analysis was used to determine whether one or more of the variables (age, sex, height, weight, and body surface area) had a predictable relationship to the left bronchial diameter. The left mainstem bronchial diameter in men and women (mean +/- SD/range) was 12.4 +/- 1.5 mm/9.5 to 15.5 mm, and 10.7 +/- 1.0 mm/9.0 to 14.0 mm, respectively. In female patients, none of the variables was significantly correlated with bronchial diameter. In male patients, age (p = 0.013) and height (p = 0.008) individually produced a statistically significant prediction of endobronchial diameter. The following formula was best predictive of left bronchial diameter in men: diameter (mm) = 0.032 x age (year) + 0.072 x height (cm) -2.043. Applying this formula to mean age 50 who are 158, 172, and 185 cm in height, the predicted left bronchial diameters are 11.0, 12.0, and 13.0 mm, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)