To investigate whether the occurrence of epithelial oedema in Fuchs' endothelial dystrophy is associated with a particular corneal thickness or the extent of central corneal guttae.Sixty-seven patients, aged 52-90 years, presenting in our clinic with Fuchs' dystrophy were divided on the basis of the presence or absence of epithelial oedema as determined by slit lamp examination. After exclusion of extreme cases, the 56 cases without oedema were compared with the 10 cases with oedema with respect to corneal thickness, measured by ultrasound pachymetry and corneal guttae diameter, obtained from the slit lamp examination.Mean corneal thickness was significantly higher (p = 0.002) in the oedematous group (mean = 0.682 mm) than in the group without epithelial oedema (mean = 0.624 mm). A corneal thickness greater than 0.650 mm was associated with a greater than 85% probability of oedema occurrence. Corneal guttae diameter did not differ significantly (p = 0.941) between the two groups and was not significantly correlated with corneal thickness (p = 0.269).There is a demonstrable association between epithelial oedema and the measured thickness of the cornea.