Mammoth Uncensored Word List:
26 - 20 letter words starting with a

abdominohysterectomy abetalipoproteinemia acetylcholinesterase acetylmethylcarbinol adrenocorticosteroid adrenocorticotrophic adrenocorticotrophin adrenocorticotropins adrenoleukodystrophy anatomicophysiologic anthropocentricities anthropomorphization antiauthoritarianism anticonservationists antienvironmentalism antienvironmentalist antifluoridationists antiinstitutionalist antiinsurrectionally antiinsurrectionists antiparliamentarians archaeometallurgical arthroophthalmopathy autolithographically autoradiographically autotransplantations