'L.A. Law' Star Larry Drake's Rare Blood Cancer Led to Death

Larry Drake -- who played Benny on "L.A. Law" -- suffered from a unique disease that caused his blood to thicken ... and ultimately contributed to his death.

According to Drake's death certificate -- he had been diagnosed with polycythemia vera -- a rare form of cancer related to an overproduction of red blood cells in the bone marrow.

The medical examiner also listed cardiac arrest, hypertension, and morbid obesity as causes of death.

TMZ broke the story ... Drake was found dead in his Hollywood apartment by a friend earlier this month.