New Zealander Kim Smith's hopes of winning the Boston Marathon were dashed overnight when she apparently injured her right leg, forcing her to give up the comfortable one-minute lead she held for the first half of the race.

The 29-year-old Aucklander, who lives and trains in Providence, Rhode Island, had been running at a pace not seen in the women's race for over a decade, gaining a lead of between 30 to 50 seconds on the pack.

But her lead was jeopardised more than halfway through the race when she appeared to stumble and struggle while running downhill.

A video posted online by CBS Boston shows Smith stopping briefly to grab her right leg, indicating a potential injury to her calf or ankle, before continuing on.

However, the pack caught up as Smith failed to overcome her injury and fell further behind. She dropped out of the race at about 32km and was treated by medics.

Smith earlier said she did not want to feel boxed in by a leisurely start and get out-kicked by the many fast African runners, including 2010 winner Teyba Erkesso of Ethiopia.

Kenyan Caroline Kilel won the women's race in an unofficial time of 2:22.36.

Smith placed fifth in last year's New York Marathon, with a time of 2:29:28.