From 1936, Löfgren worked mainly at Saint Göran Hospital in Stockholm, where he became senior physician at the pulmonary clinic in 1957.

He became interested early on in the mysterious disease morbus Schaumann, now known as sarcoidosis.

In his thesis, he showed that erythema nodosum, which had always implied tuberculosis in the past, was also present in sarcoidosis.

[2] Löfgren described how erythema nodosum, enlarged lymph nodes on the root of the lung (called hilar lymphadenopathy) and extinguished tuberculin tests were symptoms of an acute but often transient form of sarcoidosis.

He became a rallying name at symposia and congresses and, in 1958, helped found the International Sarcoidosis Committee.