OBJECTIVE: We sought to determine whether hypocortisolism is associated with preterm birth, using hair cortisol as a marker of long term hypothalamic-pituitary-adrenal axis activity. STUDY DESIGN: In a prospective, matched, case-control study, 29 women who had a preterm birth at 24–36w5d gestation were compared to 29 women who delivered at term, matched for maternal age, gestational age, and ethnicity. Cases’ samples were collected within 72 h of preterm birth and controls at the same gestational age as the corresponding case. Participants completed validated questionnaires regarding general stress and childhood trauma. The Wilcoxon signed-rank test was used to compare the distribution of mean hair cortisol scores between cases and controls. Conditional logistic regression was used to predict case vs. control by hair cortisol score, controlling for relevant covariates. RESULTS: Baseline characteristics of cases and controls did not differ. Hair cortisol levels were significantly lower among cases in the adjusted analysis. Hair cortisol level was a predictor of case versus control. Each 10-pg.mg–1 increase in hair cortisol level was associated with an estimated 33% decreased odds of being a case. The only significant difference in the validated questionnaires was an increased measure of emotional neglect in the preterm group. CONCLUSION: Our study suggests that women who deliver prematurely may have lower hair cortisol levels than women who deliver at term. Normal hypothalamic-pituitary-adrenal axis activation is a physiologic, adaptive response to stress. One hypothesis to explain our results are that women who are stressed, but unable to mount an adequate stress response could be at particular risk for preterm birth.