OBJECTIVE: To investigate the association between cone depth of the loop electrosurgical excision procedure (LEEP) of the cervix and subsequent risk of spontaneous preterm delivery. METHODS: The study included all deliveries in Denmark over a 9-year period, 1997-2005, with information obtained from various public health registries. Of the 552,678 singleton deliveries included in the study, 19,049 were preterm and 8,180 were subsequent to LEEP. Of the 8,180 deliveries with prior LEEP, 273 were subsequent to two or more LEEPs. Of the deliveries subsequent to only one LEEP, we extracted information about cone depth on 3,605 deliveries, of which 223 were preterm (6.2%). Logistic regression analyses were used to evaluate association between cone characteristics and the subsequent risk of preterm delivery, with simultaneous adjustment for potential confounders. RESULTS: Increasing cone depth was associated with a significant increase in the risk of preterm delivery, with an estimated 6% increase in risk per each additional millimeter of tissue excised (odds ratio 1.06, 95% confidence interval 1.03-1.09). Severity of the cone histology and time since LEEP were not associated with the risk of preterm delivery. Having had two or more LEEPs increased the risk almost fourfold for subsequent preterm delivery when compared with no LEEP before delivery, and almost doubled the risk when compared with one LEEP before delivery. CONCLUSION: Increasing cone depth of LEEP is directly associated with an increasing risk of preterm delivery, even after adjustment for several confounding factors. LEVEL OF EVIDENCE: II.