AIMS: To characterise changes in ocular dimensions after combined cataract operation and trabeculectomy with mitomycin C using separate incisions (combined operation). METHODS: 24 consecutive eyes that had combined operation and 16 eyes that had cataract operation alone were enrolled. The axial lengths before and after operations were determined with non-contact optical coherence biometry. The intraocular pressures (IOP), axial lengths, corneal curvatures, and the expected and observed refractive errors before and after operations were compared. RESULTS: After a combined operation, mean IOP was significantly reduced from 16.6 (SD 5.8) mm Hg to 10.9 (4.1) mm Hg (p<0.00001), and mean axial length was significantly shortened from 24.10 (0.98) mm to 23.98 (0.96) mm (p<0.00001). The mean axial length reduction after combined operation (117 (57) microm) was significantly larger than the reduction after cataract operation alone (75 (38) microm, p<0.02), and correlated significantly with the postoperative IOP (p<0.002). There was a mean with the rule surgically induced corneal astigmatism of 0.44 (0.83) dioptre by vector analysis, and a significant increase of mean keratometry reading of 0.23 (0.46) dioptre after a combined operation. However, there was no significant difference between the expected and observed refractive errors. CONCLUSIONS: Despite an alteration of the axial length and corneal curvature, the refractive outcome after a combined operation did not differ significantly from the predicted refraction.