Intravenous heparin therapy is often used in patients presenting with transient ischemic attack (TIA) or stroke as either bridging therapy for anticoagulation with warfarin, or as primary therapy in suspected intracranial arterial dissection, crescendo TIAs, or suspected hypercoagulable states. We examined the use of a bolus of intravenous heparin at the start of anticoagulation during hospital admission for patients with TIA or stroke. A subgroup analysis of a prospective, single-blinded, randomized clinical trial was undertaken to examine the effect of providing an intravenous bolus of heparin prior to continuous intravenous maintenance heparin therapy. Pre-treatment clinical factors were comparable between subgroups. Thirty-three patients received a bolus at initiation of therapy and 173 patients did not. Patients receiving a bolus had a significantly higher first activated partial thromboplastin time at 6 h after initiation of therapy than patients without bolus (87.6 +/- 36.3 versus 61.0 +/- 8.1 s). Patients receiving bolus achieved an initial activated partial thromboplastin time greater than the minimum threshold for the therapeutic range of anticoagulation (> 60 s) sooner than patients without bolus (9.6 +/- 7.3 versus 14.5 +/- 10.8 h), but did not have a significantly greater chance of achieving therapeutic range (60-90 s). The fraction of time during which anticoagulation was therapeutic was similar between patients receiving bolus or not. There was no significant difference between the number of supratherapeutic coagulation results, total dosage of intravenous heparin received, complications due to anticoagulation, nor the times required for discontinuation of heparin and discharge from hospital between subgroups. The use of an intravenous heparin bolus during initiation of anticoagulation for TIA or stroke does not appear to be associated with greater risks and can achieve a minimum therapeutic range faster than therapy without heparin bolus.