Through October 31, 2015, American Airlines and Marriott Rewards are offering a 20% bonus when you convert Marriott points to AAdvantage miles.
To qualify for this AAdvantage® bonus mile offer, member must initiate the AAdvantage mileage conversions with the Marriott Rewards® program between 12:00 am Central time October 1, 2015 and 11:59 pm Central time October 31, 2015 (the “Promotion Period”), and be in good standing AAdvantage Program and the Marriott Rewards Program. Members can convert multiple times during the Promotion Period.
You can find the full terms here.
For more information and a deeper analysis, see this Loyalty Lobby post.