Cap for writing instruments containing ink. The cap (1) is closed on all sides apart from an open insertion end (4) and comprises a safety ventilation device, which allows a flow of air along the cap (1), if the latter has lodged in a child's windpipe when swallowed accidentally. The safety ventilation device comprises an axial ventilation path (3, 6) on the outside of the cap, which along at least one part of its axial extent has an open cross-sectional configuration, so that a covering placed around the cap (1), corresponding to the windpipe, is prevented from pressing along the entire periphery of the cap (1) against its surface. On the contrary, at any axial point of the cap (1), a free cross-sectional area remains for the ventilation with a throughflow capacity which corresponds to the air requirement of a person. The ventilation path may comprise an axial perforation (3) in the base region (12) of a retaining clip (2) and the gap (6) between the retaining clip (2) and outer surface of the cap (1). It may also be formed by ribs extending axially and projecting radially from the outer surface of the cap (1) or by axial grooves in the surface of the cap. The cap (1) effectively prevents drying up of the tip (10) of the writing instrument (8) supplying ink.