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Incomparability issues, ranging from, among other things, changing consolidation perimeters to increasing accounting rules choices, plague companies’ financial analysis. To circumvent these difficulties, rules that strongly encourage the widespread use of pro-forma figures have been set up. This enables, in principle, fair comparisons between successive periods for a given company. However, the guidelines are not very stringent, as well in French GAAP, as in IFRS, and the topic is not well known. Furthermore, the reality of the studied cases exhibits a plethora of practices, as well as a paradoxical data incomparability. We will try in this paper to define a not well known topic and reveal the practices of the’SBF 250’ index companies. This work highlights how different are the use and release of socalled comparative data (like’pro forma’), and underlines that pro forma data are often discretionary disclosures when companies change their consolidation perimeter. The contribution of IFRS is discussed. We establish some research paths for broader investigations.