Is it better to improve still further at something you do best or instead choose «not to put all your eggs in the same basket?» Do or delegate: is it better to control the whole of the chain of production or to concentrate on a number of key activities and subcontract the least strategic links in the chain? These choices determine the make-up of a group's business portfolio and characterise its development mode: diversification or refocusing. The strategy which groups choose can vary over time and depending on the actors involved. The response to these questions may, in particular, not necessarily be the same from the points of view of directors and shareholders. The increased role of the financial markets appears to be encouraging groups to focus on their core businesses. Is this the strategy which has been adopted in France in recent times? It is difficult to construct statistical indicators to measure this phenomenon because activity categorisations do not effectively pick up the notions, such as «core business», which govern refocusing decisions. This article aims to overcome this observation of the poor correspondence between categorisations and concepts by suggesting two improvements to be made to the measurement tools. Firstly, by isolating support activities ­ or functional activities ­ whose development has not genuinely been diversified. Next, by taking into account the level of coherence between the group's different activities: the diversity of the group is reduced when its activities are frequently combined in a similar way in other groups. The analysis focuses on nearly 1500 groups with over 500 employees. It seems at this stage that, with the exception of functional activities, groups are not particularly diversified in France and that average diversity changed little between 1993 and 2000: some groups diversified, while others refocused. Support activities, which were already very well represented in 1993, increased in percentage terms during the p