Wetzlar began collecting in the 1930s in Amsterdam, primarily assembling paintings of the Dutch Golden Age. Under the influence of art historian Max Friedländer, Wetzlar gradually extended his collection to include paintings from other Northern schools. Friedländer wrote a preface to a catalogue of Wetzlar's collection, published in 1952. Wetzlar's collection was dispersed at auction following his death.
Friedländer, M.J. Collection Dr. H. Wetzlar, Amsterdam. Amsterdam, 1952
Sotheby, Mak van Waay. The Collection of the late Dr. Hans Wetzlar. Amsterdam, 9 June 1977.