Together with his father he co-authored many works before his father's death in 1925, most substantial of which were four volumes of monographs on the Uredinales (now called Pucciniales), Monographia Uredinearum seu specierum omnium ad hunc usque diem descriptio et adumbratio systematica (Monograph on the Uredinales, description of known species and outline of systematics).
The third volume described the systematics and taxonomy used to classify the family and provided a key as well as further descriptions of other genera including Gymnosporangium and Phragmidium.
[3] The final volume published covered the related genera; Peridermium, Aecidium, Monosporidium, Roestelia, Caeoma, Uredo and Mapea.
[4][5] Hans Sydow was a prolific author (or coauthor) of new fungal species, having formally described 3451 in his career.
[12] Several taxa of fungi were named in his honour;[13] A subspecies of Bryum capillare received the suffix "sydowii" by Czech botanist, Josef Podpěra.