People over the age of 10 years can lose their fusion ability. This acquired disruption of fusion gives rise to intractable diplopia without suppression. An involuntary vertical bobbing movement of the non-fixing eye, present only with both eyes open, occurs at or near the angle of superimposition. This appears to be a unique and characteristic sign of acquired disruption of fusion and was present in all cases. Other causes of diplopia are mentioned and differentiated. Acquired central fusion disruption usually results from serious head injury. Partial recovery may occur but is unlikely. Sensory deprivation of at least 31/2 years' duration due to poor vision in one eye resulting from a traumatic cataract and sometimes subsequent uncorrected unilateral aphakia caused loss of fusion in 15 patients. The practical implications with regard to intraocular lenses and unilateral aphakia is discussed. Less commonly, vascular, neoplastic and presumed inflammatory lesions in the mid-brain area cause central fusion disruption.