How best to manage women who are found on cervical screening to have mild dyskaryosis remains controversial. Cytological surveillance misses some lesions picked up by colposcopy, but colposcopy is emotionally traumatic for women, the majority of whom will have a normal result. To determine what proportion of lesions are missed by cytological surveillance, and whether any abnormalities persist after colposcopy, we studied, by means of colposcopy and biopsy, the prevalence of cervical intraepithelial neoplasia (CIN) and subclinical human papillomavirus infection (HPVI) in two groups of patients who had had a first smear showing mild dyskaryosis at least 24 months earlier. One group was recruited from a centre practising cytological surveillance, with colposcopy for patients showing persistent or progressive cytological abnormality. The other group all had early colposcopy and treatment. Of 214 patients recruited into the cytological surveillance group, 70 (33%) had been referred for colposcopy within 24 months. Colposcopy of the remaining 144 (after a mean interval of 27 months from presentation) revealed that 54 (38%) were disease free, 64 (44%) had HPVI/CIN1, 8 (6%) had CIN2, and 18 (12%) had CIN3. A smear at that time identified 12 of the 18 (67%) with CIN3 as needing close cytological follow-up (1 patient) or prompt referral for colposcopy (11 patients). 137 women in the early colposcopy group attended for the study colposcopy (after a mean interval of 32 months from presentation). 37% were found to have some abnormality persisting after an earlier colposcopy, but none had CIN3. Cytological surveillance of mild dyskaryosis resulted in a 12% risk of patients having a small CIN3 lesion after 2 years, but this risk was reduced to 4% by the addition of a third repeat smear 12 months after the second. With such a policy only about a third of women would require colposcopy, and the risk of missing serious underlying precancerous changes would be low.