All 20th-century chronologies of the collector’s activity of Bishop Josip Juraj Strossmayer (1815-1905) and overviews of the evolution of today’s Strossmayer’s Gallery of Old Masters at the Croatian Academy of Sciences and Arts mention the bishop’s cooperation with the Austrian Nazarene painter Leopold Kupelwieser (1796-1862), father of Paul Kupelwieser, the former owner of the Brijuni islands. This episode from the “prehistory” of Strossmayer’s Gallery has hitherto been known only as a brief notice repeated in almost identical formulations: “In 1857, the bishop sent the first larger group of paintings to Vienna in order to be restored under the supervision of painter Leopold Kupelwieser.” Research of archival documents mentioning the cooperation between Bishop Strossmayer and painter Kupelwieser has now been complemented with an overview of Kupelwieser’s activity in Croatia, with an aim of promoting the preservation and evaluation of this segment of his painting oeuvre. Besides paintings ordered by Strossmayer (presently at the Diocesan Museum of Zagreb), Kupelwieser produced two paintings for Croatian churches independently of his cooperation with the bishop (for the church of St Stephen of Hungary, today’s church of the Immaculate Conception of the Blessed Virgin Mary in Nova Gradiška, and for the chapel of St Peter and Paul in Dvor na Uni). Two more paintings are preserved on the Brijuni islands that do not directly belong to Kupelwieser’s oeuvre yet are closely linked to him.