AIMS: To investigate whether pharmacological mydriasis influences interobserver agreement or within-observer agreement (comparing estimates made before and after dilation) in the evaluation of the optic nerve head (ONH) of the glaucoma suspect or patient. METHODS: Monoscopic disc photographs of the ONH were assessed by each observer on two separate occasions in order to establish baseline intra- and inter-observer agreement. Then the ONH of 53 eyes of 53 patients was examined by each observer on two separate occasions, the pupil being pharmacologically dilated on only one of these visits. Each observer commented on the vertical and horizontal cup-to-disc (C/D) ratio, and the presence of the following ONH parameters: laminar dots; disc haemorrhage; disc saucering; disc notching, and peripapillary atrophy (PPA). RESULTS: Intersessional variability of C/D ratio estimates, based on photographs of the ONH, was similar for the two observers. The mean (+/-standard deviation) age of the 53 patients was 70 (+/-15) years, and the male : female ratio was 28 : 25. Intraobserver agreement of C/D ratio estimation performed through a dilated pupil on one occasion and an undilated pupil on the other occasion was not statistically different between observers. Interobserver agreement of C/D ratio estimates were not adversely affected in a statistically meaningful way if ONH evaluation was performed by each observer under conditions of nonmydriasis or by each observer under conditions of pharmacological mydriasis. Within-observer (before and after dilation) and interobserver agreement with respect to the presence of laminar dots (kappa=0.41-0.69), PPA (kappa=0.36-0.54), and pathological cupping (kappa=0.46-0.68) was typically moderate to good across the two visits where the pupil was dilated on one occasion only, but less reliable for saucering, disc notching and disc margin haemorrhage. CONCLUSION: Routine pharmacological mydriasis is not essential for reproducible evaluation of the ONH for patients in whom a satisfactory view of the optic disc can be achieved through the undilated pupil.