The intra-ocular pressure immediately before glaucoma surgery can be raised. We wished to investigate if ocular compression for 20 min before a combined peri- and retrobulbar injection would result in a lower pressure after the block. Sixty consecutive patients scheduled for filtration surgery were randomly assigned to receive ocular compression using an external pressure device for 20 min before combined peri- and retrobulbar injection (intervention group, who also received compression after the block) or to a control group in whom pressure was applied only after the block was completed. The intra-ocular pressure was measured at baseline, after the 20-min pre-injection compression (intervention group), after injecting the block, and after the 10-min post-injection compression. The pressure did not differ between groups at baseline, after the block or after the post-injection compression. In the intervention group, the compression before the block reduced the median (IQR [range]) pressure from 21.0 (17.0–25.0 [12.0–40.0]) mmHg to 16.8 (12.5–22.5 [7.5–33.5]) mmHg (p < 0.001). We conclude that external ocular compression reduces the intra-ocular pressure, but applying an additional compression for 20 min before injecting the block is not beneficial.