To estimate the association between cytokine levels in twin pregnancies and risk of spontaneous preterm delivery, including the effect of progesterone treatment. This secondary analysis of a randomized placebo-controlled trial investigating the effect of progesterone treatment on preterm delivery in twin pregnancies included 523 women with available dried blood spot samples collected before treatment with progesterone (n=258) or placebo (n=265) and after 4-8 weeks of treatment. Samples were analyzed for cytokines using a sandwich immunoassay. Cytokine levels in spontaneous preterm delivery at 34-37 weeks of gestation and spontaneous preterm delivery before 34 weeks of gestation were compared with delivery at 37 weeks of gestation or more for placebo-treated women. The association between interleukin (IL)-8 and risk of spontaneous preterm delivery before 34 weeks of gestation was estimated further, including comparison according to treatment. Statistical analyses included Kruskal-Wallis test, Mann-Whitney U test, linear regression, and Cox regression analysis. We found a statistically significant association between IL-8 and spontaneous preterm delivery. At 23-33 weeks of gestation, the median IL-8 level was 52 pg/mL (interquartile range 39-71, range 19-1,061) for term deliveries compared with 65 pg/mL (interquartile range 43-88, range 14-584) for spontaneous preterm delivery at 34-37 weeks of gestation and 75 pg/mL (interquartile range 57-102, range 22-1,715) for spontaneous preterm delivery before 34 weeks of gestation (P<.001). Risk of spontaneous preterm delivery was associated with a large weekly increase in IL-8 (hazard ratio 2.0, 95% confidence interval [CI] 1.2-3.3). There was no effect of progesterone treatment on IL-8 levels. Levels of IL-8 at 18-24 weeks of gestation were associated with a cervix less than 30 mm (odds ratio 1.8, 95% CI 1.2-2.7). Risk of spontaneous preterm delivery before 34 weeks of gestation is increased in women with high IL-8 levels. Progesterone treatment does not affect IL-8 levels. EudraCT, , 2006-000503-41, and ClinicalTrials.gov.