This study was conducted to assess the effects of long-term Se administration on the regression and metabolic status of patients with cervical intraepithelial neoplasia grade 1 (CIN1). This randomised, double-blind, placebo-controlled trial was carried out among fifty-eight women diagnosed with CIN1. To diagnose CIN1, we used specific diagnostic procedures of biopsy, pathological diagnosis and colposcopy. Patients were randomly assigned to two groups to receive 200 μg Se supplements as Se yeast (n 28) or placebo (n 28) daily for 6 months. After 6 months of taking Se supplements, a greater percentage of women in the Se group had regressed CIN1 (88·0 v. 56·0 %; P=0·01) compared with those in the placebo group. Long-term Se supplementation, compared with the placebo, resulted in significant decreases in fasting plasma glucose levels (-0·37 (sd 0·32) v. +0·07 (sd 0·63) mmol/l; P=0·002), serum insulin levels (-28·8 (sd 31·2) v. +13·2 (sd 40·2) pmol/l; P<0·001), homeostatic model assessment of insulin resistance values (-1·3 (se 1·3) v. +0·5 (se 1·4); P<0·001) and a significant elevation in quantitative insulin sensitivity check index (+0·03 (sd 0·03) v. -0·01 (sd 0·01); P<0·001). In addition, patients who received Se supplements had significantly decreased serum TAG (-0·14 (sd 0·55) v. +0·15 (sd 0·38) mmol/l; P=0·02) and increased HDL-cholesterol levels (+0·13 (sd 0·21) v. -0·01 (sd 0·15) mmol/l; P=0·003). In addition, compared with the placebo group, there were significant rises in plasma total antioxidant capacity (+186·1 (sd 274·6) v. +42·8 (sd 180·4) mmol/l; P=0·02) and GSH levels (+65·0 (sd 359·8) v. -294·2 (sd 581·8) μmol/l; P=0·007) and a significant decrease in malondialdehyde levels (-1·5 (sd 2·1) v. +0·1 (sd 1·4) μmol/l; P=0·001) among those who took Se supplements. Overall, taking Se supplements among patients with CIN1 led to its regression and had beneficial effects on their metabolic profiles.