Objective: To determine if cocaine use during pregnancy is associated with a reduction in the number or affinity of beta-adrenergic receptors in human myometrium. Methods: Myometrium was obtained at cesarean delivery of five women who reported using cocaine during pregnancy and from ten controls. Saturation binding assays were performed on the myometrial membrane fractions using [125I]-cyanopindolol to determine beta-adrenergic receptor concentration and affinity. The percentages of beta1- and beta2-adrenergic receptors were determined in three cocaine users and four control patients by performing competition binding assays using the beta2 antagonist ICI 118,551. Results were compared using unpaired Student t tests. Results: Women who reported using cocaine during pregnancy had a significantly lower mean (± standard deviation) concentration of myometrial beta-adrenergic receptors than did controls (22 ± 8 versus 52 ± 23 fmol/mg protein, respectively). There was no difference in the receptor affinity constants between cocaine users and controls (16 ± 2 pmol/JL for both groups). The percentages of beta1- and beta2-adrenergic receptors in the myometrium of the cocaine-use group and control group were similar: 86 ± 1% beta2 in the cocaine-use group and 83 ± 7% beta2 in the control group. Conclusion: Cocaine use during pregnancy may be associated with a down-regulation of beta-adrenergic receptors in human myometrium. This could result in a decreased capacity for uterine relaxation and, consequently, a predisposition to preterm labor. © 1995 The American College of Obstetricians and Gynecologists.