A 25% increase in bidimensional products (BPs) of tumor diameter has been used as a criterion for brain tumor progression. We studied intra-observer variability in measurements of BPs. Ten patients with contrast-enhancing glioblastoma multiforme underwent baseline and follow-up MR imaging. Seven observers measured BPs in various planes. Differences in BPs between scans were expressed as a percentage of baseline. This calculation was performed for both readings of the baseline and follow-up scans. Differences between change from baseline to follow-up on each reading (termed D values) were calculated for each reader (total of 196 D values). Median D value in each plane was calculated for each reader. Range of D values was 12.36-33.64% in axial plane (average 10.63%), 12.18-38.62% in coronal plane (average 26.84%) and 15.12-35.48% in sagittal plane (average 26.11%). Across all planes, 88 (45%) D values were >25%. When all imaging planes for any single observation were combined, in 76% of cases, at least one D value of >25% was seen. Based on the high degree of intra-observer variability, tumor measurements producing an increase in BP of >25% can routinely be obtained solely by chance.