A safety trocar instrument, for piercing the wall of an anatomical cavity to provide communication with the inside of the cavity, includes a tubular cannula and an elongate trocar having a sharp piercing point. The trocar is mounted for axial reciprocal movement within the cannula between a withdrawn rest position, in which the point is received within and shielded by the distal end of the cannula, a fully extended position in which the point is exposed beyond the distal end of the cannula, and an intermediate retracted position in which the point also is exposed beyond the distal end of the cannula. A retraction spring biases the trocar to its withdrawn position. A latch latches the trocar, in opposition to the retraction spring, for reciprocal movement between the fully extended and intermediate positions. A trip member couples the trocar to the latch member when the trocar moves from the fully extended position to the intermediate position, and trips the latch member when the trocar returns toward the fully extended position to unlatch the trocar and permit it to be moved to the withdrawn position by the retraction spring.