The young doctor had set out to become an internal medicine physician, and at first he doubted whether x-rays would have much value in the clinical care of patients.

The rest of Pfahler's career was defined by his focus on direct patient care applications of X-rays.

[2] After residency, Pfahler spent the early years of his medical career as a clinical professor at the Medico-Chirurgical College and as the director of the radiology departments at Philadelphia General Hospital and the Medico-Chirurgical Hospital.

The Medico-Chirurgical College merged with the University of Pennsylvania in 1916, and Pfahler became a professor and vice dean of radiology at that institution.

[3] He made advances both in diagnostic radiology and in radiotherapy, taking special interest in radiation treatment for oral and breast cancers.