AIM: To assess the association of floppy iris behaviour during cataract surgery with use of alpha-1-antagonists and diabetes mellitus. METHODS: 1842 eyes of 1786 patients undergoing phacohoemulsification surgery were prospectively enrolled. The use of commonly prescribed alpha-1-antagonists and the presence or absence of diabetes mellitus were noted. The occurrence of any of the features of the intraoperative floppy iris syndrome (IFIS) was noted by surgeons blinded to the patient's history. RESULTS: 57% of patients receiving tamsulosin showed features of IFIS compared with 1% of the non-tamsulosin group (p<0.001). Of these, more than half the patients manifested the syndrome in an incomplete form. Only 1 of the 51 patients receiving other alpha-1-antagonists had IFIS. Diabetes was also not associated with IFIS (p = 1). CONCLUSIONS: Tamsulosin is significantly associated with floppy iris behaviour during cataract surgery. But not all of these patients will necessarily show all or any features of IFIS. The floppy iris syndrome is likely to represent a continuum of severity. Various undefined factors, diabetes not being one of them, may have a contributory role. Non-selective alpha-1-antagonists are unlikely to be associated with IFIS.