AIMS: To study the epidemiology and risk factors contributing to displacement of nuclear fragments into the vitreous (DNFV) complicating phacoemulsification in the UK. METHODS: Cases were collected prospectively between March 2003 and March 2004 by active surveillance through the British Ophthalmological Surveillance Unit (BOSU). Case-control analysis of risk factors was performed by visiting 10 randomly selected centres using a total of 521 cases of uncomplicated phacoemulsification. Validation analysis to assess under-reporting was performed in a total of 13 randomly selected units. RESULTS: 610 cases of DNFV were confirmed during the reporting period. The estimated incidence of DNFV was 0.19-0.28%. The group with complications was significantly older than the control group (mean 76.8 vs 74.3 years: p<0.001). Significant preoperative risk factors were posterior synechiae (5.1% vs 2.2%), incomplete pupil dilation (59.5% vs 8.8%), pseudoexfoliation (5.6% vs 1.4%) and previous vitrectomy (7.8% vs 2.2%). Significant operative variables related to surgical experience, topical (14.3% vs 3.1%) and sub-Tenon's (51.4% vs 37.2%) anaesthesia, and requirement for vision blue (trypan blue ophthalmic solution) (13.7% vs 2.4%). CONCLUSIONS: The estimated incidence of DNFV during phacoemulsification surgery in the UK is two or three per 1000 operations. Risk factors have been identified that should help to guide case selection for phacoemulsification surgery and modify techniques.