A method of dynamically avoiding defective cache lines in a cache used by a processor of a computer system is disclosed. A repair mask is used, having an array of bit fields each corresponding to a cache lines in the cache, and certain bit fields in the repair mask array are initially set to indicate that a group of corresponding cache lines are defective. Thereafter the repair mask is updated by setting additional bit fields in the repair mask array to indicate that an additional group of corresponding cache lines are defective. Access to all defective cache lines is prevented based on the corresponding bit fields in the repair mask array. The initial setting of certain bit fields can take place at fabrication of the cache chip in response to testing of the cache lines. Additionally, the repair mask may be updated each time the computer system is booted in response to testing by the boot procedure. The repair mask may also be updated real-time during program execution in response to detection of an error associated with a particular cache line. Updating in real-time can be accomplished by counting a cumulative number of errors associated with a cache line, and then identifying the cache line as being defective only after a certain number of cumulative errors has occurred.