When acute glaucoma presents in its classical form, the diagnosis is fairly simple to make. However, atypical presentations are not uncommon and the diagnosis can be missed. This inevitably leads to a delay in treatment which can permanently compromise visual function. We have studied which doctors are most likely to make a misdiagnosis and calculated the resulting delay in treatment. All patients with acute glaucoma attending Whipps Cross eye unit between 1991 and 1994 were identified and their notes obtained. Thirty-eight patients were found to have had a diagnosis of acute closed angle glaucoma. In only 39.5% was the diagnosis correctly made by the referring practitioner and a misdiagnosis resulted in a mean delay of treatment of 5.8 days. General practitioners were the most likely group to have difficulty making the diagnosis, while casualty officers were most likely to make the correct diagnosis. Because acute glaucoma can present without its typical features, there needs to be a high index of suspicion for this diagnosis. This diagnosis does not require expensive equipment or a high level of training all our patients had a significant reduction in vision. Any patient who has a red eye and a subjective or objective reduction in vision should be referred to an ophthalmologist the same day.