Objective. To examine associations between first-trimester plasma cytokines and spontaneous preterm delivery (sPTD). Design. A case-control study was nested within the Danish National Birth Cohort, a cohort of women with 101,042 pregnancies from 1997 to 2002 who were recruited during pregnancy and followed prospectively. Sample. Subjects included 107 women delivering singleton infants at 24–29 weeks, 353 at 30–33 weeks, 422 at 34–36 weeks, and 1,372 at ≥37 weeks. Methods. Maternal plasma interleukin (IL)-2, IL-6, tumor necrosis factor (TNF)-α, interferon (IFN)-γ, and granulocyte-macrophage colony-stimulating factor (GM-CSF) were measured at a median of eight weeks gestation using multiplex flow cytometry. Adjusted odds ratios (ORs) were obtained using polytomous logistic regression. Main outcome measures. sPTD categorized as: 24–29 weeks, 30–33 weeks, 34–36 weeks, and ≥37 weeks (controls). Results. Elevated TNF-α and GM-CSF were associated with an increased risk of delivery at 34–36 weeks. In underweight women, sPTD <34 weeks was associated with elevated (>75th percentile) IL-6 (OR = 5.62, 95% confidence interval (CI): 1.73, 18.26) and TNF-α (OR = 3.02, CI: 1.02, 8.91) compared with term delivery. Conversely, among obese women, elevated IL-2 (OR = 0.30, CI: 0.11, 0.78) and TNF-α (OR = 0.15, CI: 0.05, 0.47) were associated with a reduced risk of delivering at <34 weeks. Cytokines were not related to delivery at <34 weeks in normal-weight and overweight women. Conclusions. These findings suggest that the association between first-trimester plasma cytokine levels and sPTD may depend on pre-pregnancy body mass index.