Preferential looking (PL), as a technique for assessing visual acuity, was designed for use with young infants. There are occasions when a practitioner may need to use a PL test with an adult who is unable to participate in acuity measures with conventional optotype tests (such as an adult with learning disabilities, stroke damage or dementia). In preparation for the development of an adult-appropriate PL test, this study compared scores with the Cardiff Acuity Test (CAT, which was designed as a PL test) and a standard LogMAR-based test, the Lea chart. One hundred and one adults with learning disabilities, attending vision screening at Special Olympics, took part in acuity measures with both tests. Athletes subsequently found to have uncorrected refractive errors were excluded and analysis was confined to 72 athletes. There was no significant difference in mean acuity with the two tests, but CAT tended to overestimate scores for poorer acuities. CAT was less sensitive than the Lea chart to interocular acuity differences. Although CAT may be a successful way to assess acuity in patients unable to participate in conventional acuity tests, the practitioner should be cautious in interpreting results. There is the potential with CAT to overestimate acuity and to miss small interocular differences, so criteria for determining 'abnormality' may need to be adjusted.