Malignant (ciliary block) glaucoma occurred in a patient 16 years after successful filtering surgery for glaucoma. Initially a moderate rise in intraocular pressure (IOP) occurred despite a functional filtering bleb. Pilocarpine treatment was started and exaggerated the attack of malignant glaucoma. The possibility that malignant glaucoma had occurred 16 years after successful filtering surgery was not appreciated until the anterior chamber deepened and a fall in IOP occurred with instillation of cycloplegic drops into the eye.