BACKGROUND: Postthrombolysis brain haemorrhagic transformations (HT) are often categorized with the CT-based classification of the European Cooperative Acute Stroke Study (ECASS). However, little is known about the reliability of this classification and its extension to MRI. Our objective was to compare the inter- and intraobserver reliability of this classification on CT and 3 MRI sequences. METHODS: Forty-three patients with postthrombolysis HT on CT or at least 1 of the 3 MRI sequences: fluid-attenuation inversion recovery (FLAIR), diffusion-weighted imaging (DWI), and T2* gradient recalled echo (T2*GRE) were selected. Twelve control patients without any bleeding were added to avoid a bias based on a pure HT-positive cohort. Each series of images were independently classified with the ECASS method by 6 blinded observers. Inter- and intraobserver reproducibility was categorized from poor to excellent depending on kappa values. RESULTS: The inter- and intraobserver overall concordance of the classification was good for T2*GRE, DWI and CT (kappa > 0.6) and moderate for FLAIR (kappa < 0.6). The interobserver concordance for parenchymal haematomas was excellent for T2*GRE (kappa > 0.8) and moderate for CT, FLAIR and DWI. CONCLUSION: The T2*GRE sequence is the most reproducible method to categorize postthrombolysis HT and has an excellent reliability for the severe parenchymal haematoma category, suggesting that this sequence should be used to assess HT in thrombolytic therapy trials.