Mammoth Uncensored Word List:
15 - 20 letter words starting with h

histocompatibilities histomorphologically homotransplantations hydrochlorothiazides hydrodesulfurization hydrometallurgically hyperadrenocorticism hypercholesterolemia hypercholesterolemic hypercoagulabilities hyperconsciousnesses hyperdolichocephalic hyperparathyroidisms hypersensitivenesses hypersynchronousness