The principal approach to the prevention of cancer of the cervix uteri has been through screening programmes,using the cervical smear (Pap test) to detect precursor lesions. The sensitivity and specificity of Pap smears dependon the skill of the observer in recognizing and classifying a variety of cellular abnormalities. We have studied thereproducibility of cytological diagnosis, according the Bethesada classification, made by cytologists in Khon Kaen,north-east Thailand, and in Helsinki, Finland, on smears taken from rural women undergoing screening during1994-2001. A total of 313 slides were reviewed. The prevalence of abnormalities was relatively high, since the seriesincluded smears judged abnormal in Khon Kaen or from women who developed cancer during follow-up, as well asa group whose smears were negative.In general, the reviewing cytologist in Finland evaluated more slides as abnormal than in the initial report. Thelevel of agreement between the two observers was evaluated by calculating the coefficient of concordance (Kappa).The kappa score depended upon the degree of detail in the diagnosis; it was 0.43 for the presence or not of anepithelial abnormality (the General Categorization of the Bethesda system), and rather higher (0.5) for separatinglow grade from high grade (HSIL or worse) abnormalities or glandular lesions. Agreement was only fair (0.37) whenthe more detailed Bethesda categories (seven) were used.The reproducibility of cervical cytology evaluations is critical to the success of screening programmes, and in thisprogramme in a moderate-high risk population of women in rural Thailand, we found that agreement betweenskilled observers, at the level of tests requiring diagnostic follow-up or not, was only moderate. The women in thisstudy are being traced to evaluate the true sensitivity of screening in terms of the lesions found on histology, duringa prolonged follow up of 4 or more years.