We evaluated if the inhibitory effect of 17alpha-hydroxyprogesterone caproate (17P) on cervical ripening is mediated by cervical proinflammatory agents. Women with singleton pregnancy and intact membranes, between 25 and 33 weeks + 6 days, were randomly allocated either to observation (22 cases, controls) or to receive 341 mg of intramuscular 17P (23 cases, 17P group), twice a week, until 36 weeks. Just before randomization, 7 and 21 days later, a cervical swab for interleukin (IL)-1beta, IL-6, IL-8, tumor necrosis factor alpha (TNF-alpha), and nitrates/nitrites (NOx) assays was collected. Moreover, an ultrasound measure of cervical length (CL) was performed at the same time. At randomization, both groups of women showed similar levels of cervical ILs and NOx. In the 17P group, cervical IL-1beta levels were significantly decreased at day 21 ( P = 0.036); in controls, they remained stable throughout the observation period. There was no significant change in IL-6, IL-8, TNF-alpha, and NOx in either group. Women in the control group had a progressive CL shortening until day 21 (median shortening of 4 mm), and this shortening was significantly less in the 17P group (median shortening of 2 mm; P = 0.017). In patients at risk of preterm labor, high-dose 17P simultaneously inhibits both cervical proinflammatory IL-1beta secretion and the progressive shortening of the cervix.