BACKGROUND: Previous studies have failed to detect high body mass index (BMI) as a risk factor for difficult tracheal intubation (DTI). BMI was investigated as a risk factor for DTI in patients planned for direct laryngoscopy. METHODS: A cohort of 91,332 consecutive patients planned for intubation by direct laryngoscopy was retrieved from the Danish Anesthesia Database. A four-point scale to grade the tracheal intubation was used. Age, sex, American Society of Anesthesiologists physical status classification, priority of surgery, history of previous DTI, modified Mallampati-score, use of neuromuscular blocker, and BMI were retrieved. Logistic regression to assess whether BMI was associated with DTI was performed. RESULTS: The frequency of DTI was 5.2% (95% confidence interval [CI] 5.0-5.3). In multivariate analyses adjusted for other significant covariates, BMI of 35 or more was a risk for DTI with an odds ratio of 1.34 (95% CI 1.19-1.51, P < 0.0001). As a stand alone test, BMI of 35 or more predicted DTI with a sensitivity of 7.5% (95% CI 7.3-7.7%) and with a predictive value of a positive test of 6.4% (95% CI 6.3-6.6%). BMI as a continuous covariate was a risk for failed intubation with an odds ratio of 1.031 (95% CI 1.002-1.061, P < 0.04). CONCLUSIONS: High BMI is a weak but statistically significant predictor of difficult and failed intubation and may be more appropriate than weight in multivariate models of prediction of DTI.