To evaluate the spontaneous regression rate (SRR) of cervical intraepithelial neoplasia I (CIN I) according to age and the presence and type of HPV infection. This was a retrospective study of 161 women primarily diagnosed with CIN I on punch-biopsy of uterine cervix and followed up without excision at least for 6 months. Spontaneous regression was defined as negative or reactive cellular change in follow-up Papanicolau (Pap) smear results. SRRs according to the presence of high-risk HPV infection and type of HPV were compared. The best age cut-off value of 34.5 years for 6-month pap abnormalities was determined using receiver operation characteristic curve. Mean age was 41 years (range, 17 to 72 years). Median follow-up period was 22 months (range, 6 to 112 months). Eighty had the result of initial HPV test, and 62 (77.5%) were positive for high-risk HPV infection. SRRs at 6-month, 1-year, and 2-year follow-up were 82.4%, 85.7%, and 87.6%, respectively. There were 10 who showed spontaneous regression at 6 months and recurrent abnormal Pap results at 1 year, all of whom showed negative results in follow-up Pap tests or biopsies. Old age was associated with higher SRR at 6 months of follow-up (p=0.001) and lower infection rate of high-risk HPV than younger counterpart (p=0.009). Nevertheless, neither high-risk HPV infection (p=0.104) nor HPV type 16 or 18 infection (p=0.172) was significantly associated with abnormal Pap result at 6 months of follow-up. The higher SRRs of older patients with CIN I might be associated with lower infection rate of high-risk HPV in these patients than younger counterpart. Primary CIN I lesion in older patients could be safely followed up without immediate ablative treatment.