Istanbul's population greatly suffered during WWI. Sanitary problems, pauperization, mortality rise deteriorated everyday life. The flows of refugees and military mobilization were major issues. Paradoxically, its Greek orthodox community increased in 1914-1918. After the Mudros Armistice (October 1918), it could hope for the Ottoman Empire's dismemberment and the return of Constantinople to the Greek fold. The events that followed and led to the birth of the Republic of Turkey would put an end to their expectations. The deterioration of everyday material life, the loyalty of Istanbul Greeks to the Ottoman State in wartime, ordinary people's position towards Hellenic irredentism are the main themes explored through the archives of Greek Orthodox parishes of the city and those of the Ecumenical Patriarchate of Constantinople.