Immediately sequential bilateral cataract surgery (ISBCS) has recently been considered effective due to faster visual rehabilitation and further improvements in quality of life. We report on the treatment course of a case of early postoperative endophthalmitis in both eyes after ISBCS. The patient was a 75-year-old woman undergoing anticancer treatment for lung cancer, who had been receiving eye drops for bilateral uveitis and secondary glaucoma since 2019. She underwent ISBCS at another hospital in 2022. In that surgery, the same instruments were used for the surgery of both eyes, without resterilization between eyes. On the night of the following day, she became aware of vision loss in both eyes, and at the examination the next day, hypopyon and corneal edema were found and she was diagnosed with bilateral postoperative endophthalmitis, and referred to our hospital. On the same day, anterior chamber irrigation, posterior capsulotomy, and vitrectomy were performed on both eyes, and the intraocular lenses were preserved. There was no recurrence of postoperative inflammation, and visual acuity was 20/30 in both eyes 6 months after vitrectomy. ISBCS is recommended to be performed with complete aseptic separation of the patient's two procedures. This important rule was not followed in this case. ISBCS should be performed in accordance with accepted protocols, such as those of the International Society of Bilateral Cataract Surgeons, the Royal College of Ophthalmologists, and the Canadian Ophthalmological Society.