We have studied the role of serum C-reactive protein determination in the diagnosis of acute pelvic inflammatory disease. Acute-phase serum C-reactive protein concentration reflected the extent and the severity of pelvic inflammatory disease more closely than erythrocyte sedimentation rate or white blood cell count determinations. We recommend that both C-reactive protein concentration and erythrocyte sedimentation rate should be routinely used to augment the clinical diagnosis of pelvic inflammatory disease.