The complex detection system leading to the discovery and treatment of precancerous lesions and early cancer of the uterine cervix is described in detail and discussed. By far the most difficult and underestimated component of this system is the screening and interpretation of cervical (Papanicolaou) smears. Cytologic case finding may fail because of inadequate samples, insufficient time devoted to screening, or human fatigue. Other weak points of the system, such as an inadequate clinical component, inadequate patient compliance, poor reproducibility of diagnoses, and ineffective aftercare, are also described. For example, obtaining a second smear to confirm or refute a diagnosis of cellular atypia is often a misleading practice. Although this cancer detection system has been shown to be effective in reducing the rate of morbidity and mortality from invasive cervical cancer in appropriately screened populations, there is no evidence that the Papanicolaou test has succeeded anywhere in complete eradication of this theoretically preventable disease. It is important to inform the public about the potential failures of the system and the reasons for them.