The aim of this prospective study was to compare the retinal nerve fibre layer thickness (RNFL) measurements obtained from the Cirrus optical coherence tomography (spectral domain OCT) with the Heidelberg Retinal Tomograph 3 (HRT3, confocal scanning laser ophthalmoscope). Eighty-eight normal subjects and 85 glaucoma subjects were included and had imaging performed with both the Cirrus OCT and HRT3. VFs with SITA 24-2 were also carried out at the same visit. HRT3 produced thicker measurements both globally and in all four quadrants compared to the Cirrus OCT. However, there was a significant strong correlation between the two imaging modalities for all paired measurements of global and sectoral RNFL. For all paired measurements, except for the nasal quadrant, Cirrus OCT showed higher sensitivities for glaucoma detection than HRT3 at similar specificities. The authors conclude that although the thickness measurements by the two technologies are strongly correlated, they are significantly different. Therefore, the measurements are not interchangeable when used in clinical practice.