PURPOSE To describe the prevalence of preoperative conjunctival bacteria of patients living in Madrid, who underwent cataract surgery in our hospital, without using exclusion criteria to select the patients. METHOD The preoperative conjunctival cultures of 4432 consecutive patients, in a three year period, were studied retrospectively. Samples of preoperative conjunctival cultures were selected from publications of the past 50 years. Our conjunctival bacteria frequencies were compared with those of other samples (chi-square test), in order to show that the bacterial isolation method is a determinant factor of the bacterial spectrum. RESULTS 93.5% bacteria were Gram positive and 6.5% Gram negative. Each bacteria group was present in the patients as follows: 56.9% Staphylococcus coagulase negative, 30.5% Corynebacterium sp, 6.5% Staphylococcus Aureus, 2.7% Streptococcus Pneumoniae, 5.6% other Streptococcus sp, 2.8% Haemophilus sp, 3.5% other Gram negative rods, 1.2% diplococci Gram negative and 0.5% other bacteria. Pathogen bacteria were isolated in 19.1% of the patients and 21.9% of the cultures were negative. The samples shown in publications previous to 1980 (table II), present bacteria frequencies statistically different from our study. CONCLUSIONS The conjunctival bacteria prevalence in preoperative cultures of patients awaiting cataract surgery had never been described in Madrid. By analysing other authors' results, it can be seen that bacteria prevalence is affected by the culture media used. It also seems that the size of the sample and the health status of the patients could affect this prevalence (Arch Soc Esp Oftalmol 2004; 79: 13-20).