A partial thromboplastin time (PTT) and a prothrombin time (PT) performed as routine preoperative screening on a young woman hospitalized for preeclampsia were unexpectedly and markedly prolonged. Since there was no clinical suspicion of a bleeding disorder, and since results of numerous repeat tests were normal, an artifact was suspected. The partially evacuated blood drawing (Vacutainer) tubes used on the patient's ward were found to have reduced draw in a range sufficient to produce similar prolongation of the PTT and PT in healthy individuals. False prolongation of the PTT and PT due to inadequate filling of the tubes is likely to be a frequent cause of confusion in patient care, if not appreciated and avoided.