Answer by taking a quote from the following article:

Johan August Strindberg (; Swedish: [2strInd,baerj] ( listen); 22 January 1849 - 14 May 1912) was a Swedish playwright, novelist, poet, essayist and painter. A prolific writer who often drew directly on his personal experience, Strindberg's career spanned four decades, during which time he wrote over sixty plays and more than thirty works of fiction, autobiography, history, cultural analysis, and politics.

Strindberg died shortly after the first of his plays was staged in the United States--The Father opened on 9 April 1912 at the Berkeley Theatre in New York, in a translation by Edith and Warner Oland.  During Christmas 1911, Strindberg became sick with pneumonia and he never recovered completely. He also started to suffer from a stomach cancer. The last weeks of his life were painful, and the daily papers in Stockholm reported on his health in every edition. He received many letters and telegrams from admirers across the country. He died on 14 May 1912 at the age of 63.  Strindberg was interred at Norra begravningsplatsen in Stockholm. He had given strict instructions concerning his funeral and how his body should be treated after death: only members of his immediate family were allowed to view his body, there would be no obduction, no photographs were taken, and no death mask was made. Strindberg had also requested that his funeral should take place as soon as possible after his death to avoid crowds of onlookers. However, the workers' organisations requested that the funeral should take place on a Sunday to make it possible for working men to pay their respects, and the funeral was postponed for five days, until Sunday, 19 May. According to Strindberg's last wish, the funeral procession was to start at 8am, again to avoid crowds, but large groups of people were nevertheless waiting outside his home as well as at the cemetery, as early as 7am. A short service was conducted by Nathan Soderblom by the bier in Strindberg's home, in the presence of three of Strindberg's children and his housekeeper, after which the coffin was taken outside for the funeral procession. The procession was followed by groups of students, workers, and members of Parliament, and it was estimated that up to 60,000 people lined the streets. King Gustaf V sent a wreath for the bier.

What did he die from?
pneumonia and he never recovered completely. He also started to suffer from a stomach cancer.