BACKGROUND: Endotracheal intubation is commonly performed via direct laryngoscopy (DL). However, in certain patients, DL may be difficult or impossible. The Bonfils Rigid Fiberscope® (BRF) is an alternative intubation device, the design of which raises the question of whether factors that predict difficult DL also predict difficult BRF. We undertook this study to determine which demographic, morphologic, and morphometric factors predict difficult intubation with the BRF. METHODS: Four hundred adult patients scheduled for elective surgery were recruited. Patients were excluded if awake intubation, rapid sequence induction, or induction without neuromuscular blocking agents was planned. Data were recorded, including age, sex, weight, height, American Society of Anesthesiologist classification, history of snoring and sleep apnea, Mallampati class, upper lip bite test score, interincisor, thyromental and sternothyroid distances, manubriomental distances in flexion and extension, neck circumference, maximal neck flexion and extension, neck skinfold thickness at the cricoid cartilage, and Cormack and Lehane grade obtained via DL after paralysis was confirmed. Quality of glottic visualization (good or poor), as well as the number of intubation attempts and time to successful intubation with the BRF, was noted. Univariate analyses were performed to evaluate the association between patient characteristics and time required for intubation. Variables that exhibited a significant correlation were included in a multivariate analysis using a standard least squares model. A P < 0.05 was considered significant. RESULTS: Glottic visualization with the BRF was good in 396 of 400 (99%) cases. On the first attempt, 390 patients were successfully intubated with the BRF; 6 patients required >1 attempt; 4 patients could not be intubated by using the BRF alone. These 4 patients were intubated by using a combination of DL and BRF (2 patients), DL and a Frova bougie (1 patient), and DL and an endotracheal tube shaped with a semirigid stylet (1 patient). Mean time for successful intubation was 26 ± 13 seconds. Multivariate analysis showed that decreased mouth opening (P = 0.008), increased body mass index (P = 0.011), and higher Cormack and Lehane grade (P = 0.038) predicted longer intubation times, whereas shorter thyromental distance predicted slightly shorter intubation times (P < 0.0001). CONCLUSIONS: Mouth opening, body mass index, and high Cormack and Lehane grade predict longer intubation times, as with DL. Decreasing thyromental distance predicts slightly shorter intubation times with the BRF, possibly because of a design initially optimized for a pediatric population with receding chins. These findings, along with the high success rate of BRF in this study, and the possibility of further increasing success rates by combining BRF with DL, help define the role of BRF intubation in contemporary airway management.