- 7 straight golds in the 400 meters (1984-2008).
- 9 straight golds in the 110-meter hurdles (1932-1972, no Games in 1940 and 1944).
- 8 straight golds in the 4 x 100 relay (1920-1956).
- 8 straight golds in the long jump (1924-1960).
- 8 straight golds in the high jump (1896-1928; no Games in 1916).
- 16 straight golds in pole vault (1896-1968; 1908 featured a tie of two Americans).
It's interesting to speculate about why one country seems to "own" an event over an extended period. Why, for example, has a U.S. runner won the last 7 runnings of the 400-meter dash? It has not resulted from the dominance of only a few runners, as 6 different men have accounted for the 7 golds. Only Michael Johnson (1996 and 2000) won more than a single gold.
Perhaps the fact that the 400 is the longest race run entirely in lanes has something to do with the U.S. streak. In races where runners don't have to stay in lanes, there can be physical contact between competitors and tactical issues come into play (e.g., not getting "boxed in" by a phalanx of other runners). Both of these factors can disrupt the top runners and lead to upsets. Assuming the U.S. has had the best 400-meter runners, the strict adherence to assigned lanes has presumably strengthened the Americans' hands (or feet) by reducing the upset potential.
On the other hand, the great potential for hitting hurdles in the 110-meter hurdle race and for dropping the baton (or losing time due to an awkward pass) in the 4 x 100 relay would seem to increase the chances for upsets, compared to other events. Still, the U.S. had long streaks of dominance in these two races. In recent Olympiad, however, poor baton passing has cost the American men and women potential gold on multiple occasions (here and here, for example).
Another factor that can derail one nation's dominance in an event is, of course, the emergence of another nation as a powerhouse in that event. The U.S. dry spell in the pole vault from 1972-1996 can thus be accounted for, in part, by the then-Soviet Union fielding a great group of vaulters. Led by Sergey Bubka, the only person to ever surpass 20 feet, the Soviets swept the gold, silver, and bronze pole-vaulting medals in 1988. Then, even with Bubka faltering in the '92 Games, the Soviets still won gold and silver.
To find lists of all medalists in Olympic competition, see the site Database Olympics.