Germany's largest telecoms company, Deutsche Telekom, was in the dock on Tuesday as investors claimed it misled them during the internet boom.
Deutsche Telekom was Germany's highest-profile presentation
The judge seemed to agree it had valued its properties inappropriately, but rejected some other parts of the case.
The telecoms giant faces lawsuits from 2,100 shareholders who are demanding a total of 100m euros (£70m; $130m) in damages.
It blames a global slump for an up to 75% fall in its share price.
The investors are claiming that the shares they bought for as much as 63.50 to 66.50 euros, between 1999 and 2000, were overvalued deliberately.
Presiding judge Meinrad Woesthoff said that how the company valued its properties would be the main issue in the case, which could last for many years.
Deutsche Telekom valued its properties in groups rather than individually. But as Judge Woesthoff pointed out, the real issue is whether it arrived at the wrong result by so doing.
The case is a challenging one for Germany, which is a relative newcomer to shareholder activism.
At present Judge Woesthoff and two lay assessors are deciding what to include and exclude from the court case.
The company's shareholders have become surprisingly militant
He has rejected the claim from angry shareholders that the telecoms company should have revealed it was planning to bid $40bn for US wireless operator Verizon, because disclosure could have jeopardised the deal.
Shares in Deutsche Telekom are owned by three million Germans, making it the most widely-held stock in a listed company there.
Other defendants in the case are: the German government, state-owned bank KfW Bankengruppe which sold its shares quickly, lead manager on the share issue Deutsche Bank, and former head of Deutsche Telekom Ron Sommer.
Rather than try all 2,100 cases, the court will choose 10 representative examples.