Abstract The present paper is dedicated to numerical methods of computation of the instationary availability of Boolean systems. Times for corrective repair actions as well as for preventive repair actions are supposed to be random. Lifetime of the system is also considered to be random. Moreover, random variations of the maintenance intervals are taken into account. Edgeworth expansions and Laguerre series expansions are used to compute the availability numerically. Examples are given to show the exactness of the computation methods.