OBJECTIVE: To characterize the relative contributions of the different abnormal Papanicolaou smear cytologic diagnoses in the Bethesda System to the subsequent histologic diagnosis of high-grade cervical neoplasia. METHODS: A total of 46,009 nonpregnant female members of the Kaiser Permanente Health Plan, Northern California Region, were studied prospectively. The main outcome measures included routine Papanicolaou smear diagnoses and subsequent histologic diagnosis of colposcopically directed cervical tissue specimens. RESULTS: Atypical squamous cells of undetermined significance (ASCUS) was the most common abnormal Papanicolaou diagnosis, representing 3.6% of the total number of smears. Of the total number of cases of histologically confirmed high-grade cervical neoplasia present in the population, the largest proportion (38.8%) was in women with smears showing ASCUS. Minimal abnormalities combined (ASCUS, atypical glandular cells of undetermined significance, and low-grade squamous intraepithelial lesion) were coincident with 68.6% of the cases of histologic high-grade cervical neoplasia diagnosed in this routine screening population. CONCLUSION: Recognition of the importance of equivocal and mild Papanicolaou test abnormalities in the subsequent diagnosis of high-grade cervical neoplasia emphasizes the need for accurate and cost-effective triage of the large population of women with minimally abnormal Papanicolaou diagnoses.