OBJECTIVES: To detect free radicals in phacoemulsification and aspiration procedures using electron-spin resonance and to observe the effect of ophthalmic viscosurgical devices (viscoelastic agents) on free radical intensity. METHODS: (1) A test tube containing BSS Plus (Alcon Laboratories, Inc, Fort Worth, Tex) with 1% of the spin-trapping agent, 5,5'-dimethyl-1-pyrroline N-oxide, without irrigation and aspiration, was exposed to ultrasound (100% for 20 seconds). A preparation of hyaluronate sodium (Healon [a cohesive agent that contains 1% hyaluronate sodium] (Pharmacia, Uppsala, Sweden) or Viscoat [a dispersive agent that contains 3% hyaluronate sodium and 4% chondroitin sulfate] (Alcon Laboratories, Inc)) was added to the solution to observe inhibitory effects. (2) To simulate a clinical procedure, an eye model with irrigation and aspiration of a combination of 1% 5,5'-dimethyl-1-pyrroline N-oxide and BSS Plus, 25 mL/min, as the irrigating solution was exposed to ultrasound (for 10, 20, or 30 seconds). Healon or Viscoat was injected into the anterior chamber. Free radicals were measured by an electron-spin resonance spectrometer. RESULTS: (1) A characteristic signal corresponding to hydroxyl radicals was detected. Similar inhibition by Healon and Viscoat was observed. (2) Two ophthalmic viscosurgical devices similarly suppressed the signal at 10 seconds. The inhibition by Healon ceased at 20 seconds, whereas Viscoat suppressed the signal throughout the time course. CONCLUSIONS: Phacoemulsification produces hydroxyl radicals in the anterior chamber even with irrigation and aspiration. The effect of ophthalmic viscosurgical devices on free radicals depends on the retention of the materials within the anterior chamber. CLINICAL RELEVANCE: There are complications associated with phacoemulsification.