In the Hawaiian Islands small numbers of Bryde's whales (Balaenoptera edeni) have been documented only in the Northwestern (leeward) Hawaiian Islands, and sei whales (B. borealis) have only recently been confirmed near the islands of Maui and Hawai'i. In November 2007, one Bryde's whale and two sei whale groups (including three subadults) were documented during a 7-day, systematic vessel-transect survey conducted east and northeast of O'ahu. The Bryde's whale sighting is the first in nearshore (<70 km) waters of the main Hawaiian Islands, and the two sei whale sightings are the first near O'ahu, including the first documented subadult sei whales there. The latter information suggests that Hawai'i may be a reproductive area for the endangered sei whale, whose breeding and calving ground locations remain unknown in the Pacific Ocean. Other than rare incidence, the lack of historical sightings of these two species despite many years of previous shipboard and aerial surveys off Hawai'i may be due to misidentification and/or poor sea conditions prevalent in deep, offshore windward waters of the Hawaiian Islands. We recommend conducting more offshore vessel surveys for, and biopsy sampling of, these species to clarify habitat use and current stock boundaries and numbers, information important for management of Pacific populations.