The relationship between atypical vessels seen colposcopically and dysplasia, carcinoma in situ (CIS), microinvasion, and frank invasion was studied quantitatively. No atypical vessels were found with dysplasia, but 2.8% of patients with CIS had atypical vessels. Half of the patients with microinvasion and all of the patients with frank invasion, in whom the entire zone of transformation was viewed, had atypical vessels. Eight-two percent of the patients with atypical vessels had invasion. The conclusions are: (1) Atypical vessels are not present with dysplasia and rarely present with CIS. (2) Atypical vessels may be associated with microinvasion, but are required for frank invasion to occur. (3) Because atypical vessels are usually associated with invasion, which can be in or near the field of atypical vessels, diagnosis cone biopsy should be performed if atypical vessels are seen and colposcopic biopsies do not show frank invasion. (4) Microinvasion without atypical vessels may be a localized disease.