A Spanish man was also gored in the "encierro", when runners in red scarves and white outfits dash through the Spanish town's streets pursued by the huge animals.
Chicago resident Bill Hillmann, 32, tripped and fell when a bull gored him in his right thigh.
The Spaniard, a 35-year-old man from Valencia, was gored in the chest and taken to hospital in a semi-conscious state. The two men are in a serious condition.
A further three men, all Spanish, were taken to hospital with lesser injuries from the chaotic stampede through the narrow streets of Pamplona's old town.
Hemingway aficionado Hillmann travels to Pamplona every year.