PURPOSE: To quantify and compare the in vitro adherence of Staphylococcus epidermidis and Pseudomonas alcaligenes to different intraocular lenses (IOLs). METHODS: Fourteen intraocular lenses were used in the experiment. Four of polymethylmethacrylate (PMMA), four of silicone, four of hydrogel and two of acrylic. Eight intraocular lenses were placed in eight test tubes containing 4 ml of Pseudomonas alcaligenes suspension, and six intraocular lenses were placed in six test tubes containing 4 ml of Staphylococcus epidermidis suspension. The bacterial suspension used for adherence tests was 10(8) colony-forming units per milliliter (CFU/mL) which corresponds to 0.5 in the scale of McFarland. The lenses were incubated at 37 degrees for two hours. After this, intraocular lenses were removed from the test tubes and dried twice with the use of distilled and sterile water. The material was spread on blood-agar for cultivation at 35-37 degrees C and was evaluated every 24 hours up to 72 hours. In the samples where there was bacterial growth, the colonies were counted using the conventional laboratory methods. All assays were performed in duplicate. RESULTS: Adherence of Staphylococcus epidermidis to PMMA intraocular lenses was lower than to hydrogel and silicone intraocular lenses. Adherence of Pseudomonas alcaligenes to hydrogel intraocular lenses was lower than to PMMA, acrylic and silicone intraocular lenses. CONCLUSIONS: Results suggest that in vitro adherence of Staphylococcus epidermidis and Pseudomonas alcaligenes to intraocular lenses is influenced by type of material of the lens and by microorganism species. Bacterial adherence may play a role in the pathogenesis of some forms of endophthalmitis after cataract surgery. More in vitro and clinical studies are necessary to elucidate the mechanisms by which Staphylococcus epidermidis and Pseudomonas alcaligenes cause endophthalmitis.