After another year of postgraduate study at the Max Planck Institute for Biophysical Chemistry in Göttingen, he joined the department of pathology at Johann Wolfgang Goethe University in Frankfurt-am-Main.

There, under the direction of Professor Arnold Lauche,[2] he began his lifelong study of hematopoietic diseases, with emphasis on malignant lymphomas.

This was a prestigious appointment, because at that time, 75% of department chairs at CAU were the sons of prominent Professors of Medicine in Germany.

Around 1975, Lennert began work on a system of nosological classification for malignant lymph node tumors (non-Hodgkin lymphomas) that was predicated on the cytomorphological and biochemical attributes of developing lymphoid cells.

[5] It was meant to supplement or supersede other schemes that were existence at that time, including the Lukes-Collins system [6] and the Rappaport classification.

In total, Lennert contributed over 300 scholarly papers and 4 textbooks to the medical literature during his working life.