| 100% | HYDROGEN | 8 | The H of H-bomb, first tested in the US in 1952 |
| 55% | ALI | 3 | First boxer in the US Olympics Hall of Fame |
| 52% | IVES | 4 | First major publisher of board games in the U.S. |
| 51% | BREWER | 6 | Margaret, first woman in the US Marine Corps to reach the rank of general (1978) |
| 51% | AARON | 5 | Alphabetically first of the 100 most popular boys' names in the U.S. |
| 51% | LOS | 3 | First word in the name of the U.S.'s most populous county |
| 50% | AGENTLEMANOF | 12 | & 8A 1910 P G Wodehouse novel first published in the US as The Intrusion of Jimmy |
| 49% | RCA | 3 | Developer of the U.S.'s first TV test pattern |
| 48% | SEE | 3 | Word in the first line of the U.S. national anthem |
| 48% | DAYTIMEMEAWARDS | 15 | Prizes won by Acadia National Park for the first sight of sunrise in the U.S.? |
| 47% | MARDIGRAS | 9 | Festival first celebrated in the US in Mobile, Alabama |
| 46% | ELANA | 5 | ___ Meyers, who in 2015 was part of the first world champion team from the US in women's bobsled |
| 46% | IKE | 3 | Nickname of the first U.S. president born in 7-Down |
| 46% | ELMO | 4 | Saint ___ Brady, first African American in the US to obtain a Ph.D. degree in chemistry (University of Illinois, 1916) |
| 46% | IDA | 3 | ___ B. Wells, civil-rights leader who founded one of the first Black women’s suffrage groups in the U.S. |
| 46% | BOAC | 4 | Airline mentioned in the first line of the Beatles' "Back in the U.S.S.R." |
| 46% | USN | 3 | JFK was the first president to have served in this branch of the US Armed Forces |
| 45% | LOSALAMOS | 9 | Town in New Mexico, US, where the first atomic bomb was developed |
| 45% | TCCHEN | 6 | Scorer of the first double eagle in U.S. Open history, 1985 |
| 45% | NHL | 3 | It first expanded to the US in 1924 |
| 45% | LINCOLN | 7 | Abraham ___, 16th President of the U.S.A. and the first to sport a beard in office |
| 44% | ABBA | 4 | Band whose first hit in the US was "Waterloo" |
| 44% | BARRYMORE | 9 | Surname of a famous acting family in the US |
| 44% | OBERLIN | 7 | First coeducational college in the U.S. |
| 44% | HIROSHIMA | 9 | City in Japan; target in 1945 of the first atomic bomb dropped on a populated area |
| 44% | ADAM | 4 | First human in the Book of Genesis |
| 44% | ASTOR | 5 | First woman in the House of Commons |
| 44% | PIONEER | 7 | — 10, US spacecraft launched in 1972 that made the first fly-by of Jupiter |
| 44% | ZOO | 3 | Philadelphia had the first one in the U.S. (1874) |