Severe anterior segment inflammation after cataract surgery often alerts the ophthalmologist to two main differential diagnoses – toxic anterior segment syndrome (TASS) and acute post-operative endophthalmitis. TASS is a sterile post-operative inflammatory reaction caused by a non-infectious substance that enters the anterior segment, resulting in toxic damage to intraocular tissues. Visual prognosis is usually excellent due to its responsiveness to topical steroids. Sengupta and colleagues found that the mean visual acuity (VA) was 0.11 logMAR (approximately 6/7.5 Snellen acuity) one month after treatment. Classic features of TASS include early (day zero or one post-operatively), intense, painless anterior segment inflammation without vitreous involvement. The hallmark of TASS is its rapid onset, usually within 12 to 24hours. Rarely, TASS can present as delayed onset post-operative inflammation, which canmake it difficult to distinguish from acute post-operative endophthalmitis. Acute post-operative endophthalmitis refers to a serious intraocular inflammatory disorder resulting from direct inoculation of organisms during surgery, causing an infection of the vitreous cavity. Visual prognosis is often poor. Ng and colleagues found that 52.5 per cent of patients had a VA of 6/ 60 or worse at discharge and that one-third of patients in his study had a VA of worse than 6/18 six months after admission for endophthalmitis. Significant predictors of poor visual outcome include an initial VA of light perception or worse and infection by virulent organisms such as Staphylococcus aureus, streptococcal species andGram-negative bacteria. Compared to TASS, infectious endophthalmitis often has a later presentation (peaks between days three and seven), with significant pain and vitritis. These features are summarised in Table 1. Onset, rapidity of symptomprogression and the presence or absence of pain and vitritis are the key differentiating features between TASS and infectious endophthalmitis; however, both conditions can present with poor visual acuity, corneal inflammation and significant anterior chamber reaction. Our case illustrates this diagnostic dilemma, as the patient had features suggestive of both TASS (presentation on day one post-operatively, absence of pain, sporadic occurrence) and endophthalmitis (vitritis).