In the women's competition, runners to watch include Kenya's Gladys Cherono, who won the 2015 Berlin Marathon, and Edna Kiplagat, who finished second in Chicago a year ago.
Kiplagat, the 2011 and 2013 IAAF World Champion in the marathon, finished with a time of 2:21:52.
Marcel Hug won the wheelchair race for the third time, outpushing 10-time champion Ernst Van Dyk and finishing in 1:18:04 to beat the course record and world best by 21 seconds.
With two-time fourth-place finisher Shalane Flanagan out with a back injury, the USA hopes on the women's side are represented by Desi Linden, a two-time Olympian who has thrice placed in the top 10 in Boston.
Hayle gave Ethiopia its first-ever sweep past year when he pulled away from countryman and two-time champion (2013, 2015) Lelisa Desisa with about a mile to go to join Baysa on the victory stand.
Keflezighi, 41, finished 13th this year in what he said will be his last competitive run in Boston.
Depending on their age group, runners need to get a BQ - short for Boston qualifier - at a previous marathon. "I knew if I did that no one would believe women could run distances and deserved to be in the Boston Marathon; they would just think that I was a clown, and that women were barging into events where they had no ability", she explained on her website. Fellow Swiss Manuela Schar shattered the women's mark by more than five minutes, winning in 1:28:17.More news: Carmelo responds to Phil Jackson saying he'd be 'better off' elsewhere
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Today in Boston, the running scene is celebrating women's achievements in the marathon, in running and in sports as a whole. The straight-line Boston course doesn't qualify for world records because of the possibility of a supportive tailwind like the one on Monday.
The race has largely returned to normal four years after two bombs at the finish line in 2013 killed three people and wounded hundreds more.
Sculptor Pablo Eduardo was chosen unanimously by the families to design the markers that will be placed on the two bombing sites on Boylston Street.
The elite women are on their way in the 121st running of the Boston Marathon.
Update 10:55 a.m.: Though some athletes have already completed the marathon's 26.2 miles, others haven't even started the race yet. That starts at 9:32 a.m. with the men going at 10:00 a.m. sharp.
Geoffrey Kirui, of Kenya, crosses the finish line to win the 121st Boston Marathon on Monday, April 17, 2017, in Boston.
Update 10:40 a.m.: The Boston Athletic Association announces the victor of the men's push-rim: Marcel Hug.