When times to failure of a unit follow the Weibull distribution, achievement of optimal costs under age replacement is shown to be quite insensitive to the value of the shape parameter. Moreover, it is shown that under condition based replacement considerable improvement on even optimal age replacement costs is possible for any time to failure distribution with increasing failure rate, provided that the unit possesses a measurable prognostic characteristic which predicts the moment of failure within a reasonable margin of error and a sufficient lead time to undertake (preventive) replacement before a failure actually occurs.