The aim of the study was to compare interobserver variability for The Bethesda System (TBS) and World Health Organization (WHO) classification of cervical squamous intraepithelial lesions. A total of 1,000 conventional Papanicolaou smears (156 positive and 884 negative) were examined "blindly" by three cytologists and one cytotechnician. The degree of observer agreement was expressed by kappa statistics using a program for the calculation of interobserver variation and association "Agree" (Svanholm and Jergensen, 1989). Kappa (kappa) was determined for each cytologic diagnosis within a particular classification and total for either classification. The association with and separation from other diagnoses was determined for each cytologic diagnosis in the form of conditional probability (P(j)). In WHO classification, the diagnoses of dysplasia media and dysplasia gravis showed poor reproducibility (kappa = 0.114 and kappa = 0.259, respectively), the diagnosis of dysplasia levis good reproducibility (kappa = 0.639), and the diagnosis of carcinoma in situ excellent reproducibility (kappa = 0.762). WHO classification yielded pool kappa of 0.741. In TBS classification, the diagnosis of LSIL showed good, and HSIL excellent reproducibility (kappa = 0.542 and kappa = 0.763, respectively). TBS classification yielded pool kappa of 0.699. Dysplasia media (P(j) = 0.121) and dysplasia gravis (P(j) = 0.274) were found to be morphologically poorly defined, and carcinoma in situ (P(j) = 0.777) and dysplasia levis (P(j) = 0.651) well defined diagnoses. LSIL was morphologically moderately defined (P(j) = 0.587) and HSIL well defined (P(j) = 0.789) diagnosis. Accordingly, TBS does not substantially improve diagnostic reproducibility of the cytologic diagnoses of squamous intraepithelial lesions, while providing considerably less information to the clinician than the four-grade dysplasia/CIS terminology, thus eliminating the opportunity of choosing a different procedure for the diagnosis of dysplasia media, which is of utmost importance in the population of young nulliparae.