Previous studies of cortisol concentrations in hair have concluded that it is not possible to measure more than 6 months retrospectively. This study shows for the first time that it is possible to analyze hair cortisol concentrations month-by-month for a retrospective period of 24 months. In addition, we have determined whether cortisol concentration decreases with time. The study population was 48 women in the age range 20-51 years, all with hair of length of 24 cm or longer. The participants completed a questionnaire that examined exposure to life stressors and potential confounders. Competitive radioimmunoassay was used to extract and analyze cortisol levels in hair. The overall intraclass correlation for the participants was substantial (ICC = 0.38, 95% CI 0.29, 0.49), indicating a strong within-person correlation during the growth period. The median levels of cortisol were reasonably stable. Wash-out effects were small, even for those who reported that they washed their hair every day. We conclude that it is possible to detect hair cortisol concentrations every month at least two years back in time. Changes in hair cortisol concentration are more likely to be related to life stressors than changes due to time since growth.