Three trephines were tested to determine their uniformity in creating a circular opening during penetrating keratoplasty. Twenty-one globes (seven per instrument) were trephined with a hand-held, suction or automated trephine. The mean astigmatism was greatest with the hand-held device (mean 0.73 cm) and least with the automated trephine (mean 0.50 cm). The automated and the suction trephines (mean astigmatism 0.51 cm) may result in reduced corneal astigmatism during penetrating keratoplasty.