On June 19, 1949, Valentiner died in Sønderborg hospital from lung disease, likely precipitated by the inhalation of toxic vapors from the engines in his first U-boats, U-10 and U-3.
1949. aasta 19. juunil suri Valentiner Sønderborgi haiglas kopsuhaigusesse, mille ta tõenäoliselt sai, kui hingas sisse mürgiseid aurusid, mida tekitasid tema esimeste allveelaevade U-10 ja U-3 mootorid.