Former Windows boss Steven Sinofsky's decision to leave last November marked the latest departure in a line of would-be CEOs. Sinofsky had been widely touted to succeed Ballmer for three years before he left.
Ballmer shocked markets on Friday by announcing he would step down within 12 months, ending a tenure marked by the software giant's declining dominance and struggles to keep pace with its competitors.
Over the last six years alone, Ballmer has seen off a clutch of rising stars that were discussed as potential leaders.
Windows and online head Kevin Johnson went to run Juniper Networks, Office chief Stephen Elop went to lead phone maker Nokia, while Ray Ozzie — the software guru Bill Gates designated as Microsoft's big-picture thinker — left to start his own project.
Ballmer's surprise decision follows the company's announcement last month of a major restructuring to organize itself around key areas designed to make the company more nimble in the face of fierce competition in the technology sector. The move moved nearly all of the most senior managers to new roles in the company.
Under the new structure, Julie Larson-Green, previously the co-chief of the main Windows division following Sinofsky's departure, now oversees a new division charged with all hardware devices, from the Surface tablet to the Xbox. Larson-Green is seen as one possible successor to Ballmer.
The software company said its board had appointed a special committee to direct the process of appointing a new CEO.
The committee is chaired by John Thompson, the board's lead independent director, and includes Microsoft founder and Chairman Bill Gates, as well as other board members Chuck Noski and Steve Luczo.
It will consider both external and internal candidates and is working with executive recruiting firm Heidrick & Struggles International Inc, according to the company.