Objective: Polycystic ovary syndrome (PCOS) is a common reproductive disorder characterised by insulin resistance and often associated with hirsutism. Insulin sensitising agents, such as metformin, improve both the biochemical and reproductive parameters; however, no study has been designed to specifically assess the effect of metformin on hair growth. Design and patients: Sixteen women with PCOS and hirsutism were enrolled into a 14 month (two 6 month phases with a 2 month washout) double-blind placebo-controlled cross over study. Measurements: Hirsutism was assessed using the Ferriman and Gallwey (F-G) score, patient self-assessment and growth velocity. Weight, height and waist -hip ratio were recorded. Gonadotrophins, androgens, plasma glucose and lipids were also measured. Results: Ten women completed the full 14 month study. There was a significant improvement in hirsutism at the end of the metformin phase compared with placebo: F-G score 15:8^1:4 vs 17:51:2 ðP ¼ 0:025Þ and patient self-assessment 2:4^0:1 vs 3:3^0:3 ðP ¼ 0:014Þ: Growth velocity, in millimetres per day at the end of each phase also improved ð0:67^0:17 vs 0:77^0:11; P ¼ 0:03Þ: There was a non-significant improvement in both sex hormone binding globulin (SHBG) and free androgen index (FAI), although there was a significant difference between baseline and metformin treatment for SHBG ðP ¼ 0:023Þ and FAI ðP ¼ 0:036Þ: Metformin treatment also reduced weight significantly ð91:5^7:6 vs 94:0^9:8 kg; P ¼ 0:009Þ and led to a significant improvement in cycle frequency ð0:53^0:12 vs 0:35^0:08 cycles per month; P ¼ 0:008Þ: Conclusion: We have demonstrated that metformin treatment in a group of women with PCOS results in a clinically and statistically significant improvement in hair growth compared with placebo.