Mammoth Uncensored Word List:
17 words ending with as, 16 letters long

bromomenorrhoeas cacodemonomanias choriocarcinomas counterguerillas dermatillomanias dyschromatopsias hemangiosarcomas hepatocarcinomas keratoacanthomas lymphogranulomas medulloblastomas oligodendroglias paraproteinemias phenylketonurias pleuropneumonias tachyarrhythmias teratocarcinomas