There are 9 10-letter words that start with 'dou'.
| doubleness | doubletons | doubtfully | 
| doubtingly | douchebags | doughfaces | 
| doughtiest | douppionis | dournesses | 
Note: These 9 10-letter words that start with 'dou' are found from the USA & Canada version of the tournament Scrabble word list.
Also see words start with doub, doug, dour, douc, dous, doum, doup, doul, douz, doux.
4-letter words start with 'dou' (3)