Most mail-return sterilization-monitoring services use spore strips to test sterilizers in dental clinics, but factors such as delay caused by mailing to the laboratory could cause false negatives. The aims of this study were to determine the influence of poststerilization time and temperature on the biologic indicator recovery system and to evaluate sterilization failure and its possible causes in dental clinics subscribing to a mail-return sterilization-monitoring service. Spore strips used in independent tests revealed the poststerilization time and temperature after a 7-day delay to have no significant influence. Sixty-six dental clinics that received quarterly biologic indicators to evaluate the effectiveness of their sterilizers had sterilization failure rates of 28.7% in 1992, 18.1% in 1993, and 9.1% in 1994, a statistically significant decrease in sterilization failure during the 3-year period. The usual causes of failure were operator error in wrapping of instruments, loading, operating temperature, or exposure time.