Cytologic screening for cervical cancer has proven to be beneficial in many countries, although the value of this intervention in women older than 50 years remains controversial. The purpose of this study was to investigate the efficiency of detecting cancer in situ by means of a cytologic smear at different ages, with special emphasis on the benefit of screening in women above the age of 50. We analyzed 466,275 smears taken in an open cohort of I 18.890 women in Sweden between I969 and 1988. The number of cancers in situ detected per 1.000 smears, the detection ratio, was used as an outcome measure in univariate analyses and in multivariate regression models. Cancer in situ was detected in 1,076 women in the study cohort. The detection ratio peaked at ages 30 to 34 and decreased heavily during the next I5 years of age. The efficiency of taking smears at ages above 50 was only 20 per cent (OR, 0.19; 95% CI, 0.14-0.26) of that at ages 30 to 34. These results were not changed when adjusting for time period and time interval since the previous smear. In spite of a high incidence of invasive cervical cancer in older women, the benefit of cytologic screening to detect cancer in situ above the age of 50 is uncertain.