Mammoth Uncensored Word List:
17 - 19 letter words starting with di

diastereoisomerisms diethylcarbamazines diethylstilbestrols differentiabilities dimethylnitrosamine dimethyltryptamines directionlessnesses disacknowledgements disadvantagednesses disadvantageousness disenfranchisements disinterestednesses dispassionatenesses disproportionations disrespectabilities disrespectfulnesses distinguishableness