A 53-year-old female patient presented with increased light sensitivity 3 weeks after oral intake of moxifloxacin tablets for an upper respiratory tract infection. The symptoms were anisocoria and the pupils did not react to light or accommodation. The examination of the anterior segment of the eye revealed extensive bilateral iris transillumination defects (ITD). We diagnosed a bilateral acute iris transillumination (BAIT) syndrome. The BAIT syndrome is a rare disorder associated with massive depigmentation of the iris and atrophy of the iris musculature. A risk factor for BAIT syndrome seems to be the oral intake of antibiotics, in particular moxifloxacin after an upper respiratory tract infection but cases of spontaneous occurrence have also been described. Middle-aged women are particularly affected. The exact cause of BAIT syndrome is so far unknown but a potential mechanism involves the concentration of the antibiotic in the vitreous body. Differential diagnoses include other causes for ITD, such as albinism, intraocular inflammation, pseudoexfoliation syndrome and pigment dispersion syndrome. To date there is no specific treatment for BAIT syndrome. Possible complications include increased light sensitivity and post-BAIT glaucoma. Knowledge of the rare BAIT syndrome can be useful in the clinical routine for the differential diagnostic classification of an anisocoria and can possibly contribute to avoidance of unnecessary diagnostic steps.