A special colposcopy clinic was established at the Royal Free Hospital to investigate women whose referral smears showed mild dyskaryosis. Of 200 women in the study, 66 (33%) had histologically proven CIN II or CIN III, 59 (29%) had CIN I or human papillomavirus changes, and 54 (27%) were considered normal. These findings demonstrate the importance of adequate diagnosis of this group of women. Of 143 women who had had a single mildly dyskaryotic smear, 45 (31%) had either CIN II or III. Age was not useful for predicting which women were at high risk of significant disease. Careful repeat cervical cytology correlated closely with the histological grade of the lesion. Repeat cytology was associated with an overall 24% false-negative rate, but most missed lesions were of low grade. Repeat cytology correctly identified 82% of all CIN lesions, and 93% of the most significant lesions (CIN II and III). Women who have a mildly dyskaryotic smear followed by a negative smear should not be considered normal, but careful repeat cytology can be considered a reasonably safe practice.