OBJECTIVE To determine the extent of interlaboratory variation and accuracy in the measurement of glycated hemoglobin (GHb). METHODS All laboratories that measure glycated hemoglobin in the State of Victoria, Australia, were invited to participate, and positive responses were received from 27 to 30 laboratories. An aliquot of blood drawn from three patients with diabetes and varied glycemia and from one nondiabetic subject was sent to each participating laboratory. Distribution of results was analyzed according to the reported results and their variance from an assigned reference value and were expressed as differing from this latter value as percentage bias and in absolute terms. A bias > or = 10% or an absolute difference of > or = 1% HbA1c from the reference value was considered significant. RESULTS Reported results for the same blood sample ranged from 4.1 to 5.8%, 5.1 to 8.2%, 6.7 to 9.3%, and 10.1 to 14.7% for the specimens from the nondiabetic subject and the diabetic patients with good, moderate, and poor glycemic control, respectively. The proportion of laboratories with results that differed by > or = 10% bias from the reference value were 39% (12 of 30), 29% (9 of 30), 16% (5 of 30), and 32% (10 of 30), and the proportion reporting results that differed by > or = 1% HbA1c in absolute terms from the reference values were 3% (1 of 30), 6% (2 of 30), 16% (5 of 30), and 23% (7 of 30) for the specimens from the nondiabetic subject and the diabetic patients with good, moderate, and poor glycemic control, respectively. CONCLUSIONS A substantial degree of interlaboratory variation for GHb measurement exists in Victoria, Australia. This may lead to difficulties in interpretation when GHb is assayed by different laboratories in the same patient over time. Interlaboratory standardization may be achievable by calibration to a standard assigned by a reference laboratory and distributed to all laboratories measuring GHb.