Measurement of cerebral blood flow (CBF), cerebral blood volume (CBV), cerebral oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO₂) by positron emission tomography (PET) with oxygen-15 labelled carbon dioxide (C¹⁵O₂) or ¹⁵O-labelled water (H₂ ¹⁵O), ¹⁵O-labelled carbon monoxide (C¹⁵O) and ¹⁵O-labelled oxygen (¹⁵O₂) is useful for diagnosis and treatment planning in cases of cerebrovascular disease. The measured values theoretically depend on various factors, which may differ between PET centres. This study explored the applicability of a database of ¹⁵O-PET by examining between-centre and within-centre variation in values. Eleven PET centres participated in this multicentre study; seven used the steady-state inhalation method, one used build-up inhalation and three used bolus administration of C¹⁵O₂ (or H₂ ¹⁵O) and ¹⁵O₂. All used C¹⁵O for measurement of CBV. Subjects comprised 70 healthy volunteers (43 men and 27 women; mean age 51.8±15.1 years). Overall mean±SD values for cerebral cortical regions were: CBF=44.4±6.5 ml 100 ml⁻¹ min⁻¹; CBV=3.8±0.7 ml 100 ml⁻¹; OEF=0.44±0.06; CMRO₂=3.3±0.5 ml 100 ml⁻¹ min⁻¹. Significant between-centre variation was observed in CBV, OEF and CMRO₂ by one-way analysis of variance. However, the overall inter-individual variation in CBF, CBV, OEF and CMRO₂ was acceptably small. Building a database of normal cerebral haemodynamics obtained by the¹⁵O-PET methods may be practicable.