Purpose <p>This study aimed to compare a new swept-source biometer to the gold standard B-mode ultrasound biometer for measuring the axial length (AL) when standard optical biometers failed due to cataract density.</p> Methods <p>Patients with advanced cataracts whose AL could not be measured using optical biometers available in our clinics, including the Lenstar LS-900 and IOLMaster 500 and 700, were included. The AL, anterior chamber depth (ACD), and lens thickness (LT) were measured using a new swept-source biometer (SSB) (Argos^®, Alcon) and B-mode ultrasound. The Enhanced Retinal Visualization (ERV) mode of the new SSB was used when the standard mode did not provide reliable AL measurements.</p> Results <p>AL measurements failed in 183 eyes due to cataract density using available biometers. The new SSB allowed successfully measuring the AL in 89.6% of cases, and the ERV mode was needed in 15.8% of eyes. The ALs measured with the new SSB and B-mode ultrasound were comparable, with an excellent intraclass correlation coefficient (ICC) of 0.99. No significant differences in ACD measurements were observed between new SSB and ultrasound or conventional biometers, with ICC of 0.81 and 0.87, respectively. However, the LT tended to be thinner with the new SSB compared to ultrasound, suggesting a potential source of error, but no significant difference was observed with conventional biometers (ICC of 0.88).</p> Conclusions <p>The new SSB Argos^®, in particular when the ERV mode is used, could be a reliable alternative to B-mode ultrasound for measuring AL in eyes with dense cataracts. It is as effective as ultrasound in measuring the AL, and it could help to improve cataract surgery planning and outcomes.</p>