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Aims In 1990, The Bethesda System (TBS) was introduced into spontaneous cervical screening practice in Ravenna, Italy. Negative/benign reports with the recommendation for early repeat smears (RERS) due to some limitation in sample adequacy were considered no longer acceptable. A monitoring program for the rate of unsatisfactory smears (UNS) was implemented. The aim of the present study was to evaluate the effects of such changes in the screening procedure.Methods The frequency of UNS in 1990 was compared with that of UNS+RERS in 1988 (assumed as a baseline year) by the calculation of the standardized rate ratio with the 95% confidence interval (CI). The trend in the standardized rate of UNS from 1990 to 1994 was evaluated by the calculation of the average annual variation with the 95% CI.Results The immediate effect of TBS (1990:1988 comparison) was a significant increase in the rate of UNS attributable to scant cellularity, poor fixation and thick areas (rate ratio, 2.35; 95% CI, 2.18 to 2.53) and to the absence of endocervical component (1.45; 95% CI, 1.30 to 1.60). The rate of UNS attributable to the presence of cytolysis, inflammation, blood and foreign material decreased by about 6 times (0.16; 95% CI, 0.13 to 0.19). The midterm effect of TBS (trend from 1990 to 1994) was a decrease in the total rate of UNS by an average of 2.3% per year. The downward trend was significant for smears showing scant cellularity, poor fixation and thick areas (-1.5% per year) and the absence of endocervical component (-0.7% per year). UNS attributable to the presence of cytolysis, inflammation, blood and foreign material stabilized.Conclusions TBS led to a substantial change in the type of information provided by the cytology report (Immediate effect). The monitoring program according to TBS led to a reduction in UNS attributable to sample taker (midterm effect).