As part of a case control within cohort study 472 pregnant women answered a questionnaire post partum. Serum relaxin concentrations were measured by a homologous ELISA in samples collected from the women in the 30th week of pregnancy. Serum relaxin concentrations were not associated with pregnancy-associated pelvic pain. Thus, normal pregnant women without pelvic pain (n = 118) had mean concentrations of 343 pg/ml compared to 332 pg/ml in women (n = 59) with pelvic pain and some restriction in daily activities and to 349 pg/ ml in women (n = 9) with severe pregnancy-induced pelvic pain. The present results do not suggest a role for relaxin in symptom-giving pelvic relaxation during human pregnancy.