OBJECTIVES. α-blockers are a group of α-adrenoceptor antagonists used by urologists to treat lower urinary tract symptoms suggestive of benign prostatic hyperplasia. Recent studies have suggested that these drugs - tamsulosin in particular - are involved in the development of iris complications during phacoemulsification. The objective of this study is to investigate the effects of α-blockers - especially tamsulosin - on pupil diameter. MATERIALS AND METHODS. We measured the photopic, mesopic and post-dilatation pupil diameters in both eyes of 24 patients (46 eyes in total), 16 of them treated with α-blockers and 8 of them (16 eyes in total) not treated with any drugs (controls). RESULTS. All patients treated with tamsulosin showed minor photopic, mesopic and post-dilatation diameters compared to controls. Patients treated with other α-blockers did not show any difference compared to controls. CONCLUSIONS. Even if a small number of eyes was evaluated, our study shows that tamsulosin - through its selective effect on α1A receptors - is the most involved drug in the Intraoperative Floppy Iris Syndrome.