Vladimir Nekrasov, 66, chairman of the Lukoil board of directors, died 'suddenly', with Russian state media reporting the 'preliminary' conclusion of doctors was that he suffered 'acute heart failure'.
In the end, it's always heart failure
His death follows that of Ravil Maganov, 67, at Central Clinical Hospital, also known as the Kremlin clinic, in September last year.
Officially, Maganov had been in hospital for a long-standing heart problem and fell from a sixth-floor window, dying on the spot.
On the same morning, Vladimir Putin - who had earlier decorated Maganov with a top honour - swept into the hospital to pay his final respects to Mikhail Gorbachev, the last Soviet leader, who had died the same week.
Billionaire Alexander Subbotin, 43, also linked to energy giant Lukoil where he was a top manager, was found dead in May after 'taking advice from shamans'.
One theory is that Subbotin - who also owned a shipping company - was poisoned by toad venom, triggering a heart attack.