A method is described to obtain approximate reliabilities for breeding values calculated with a multiple-lactation random regression test day model. The method was based on a concept of an equivalent number of progeny for animals with records, which is further used to derive reliabilities for related animals. The procedure accounted for the size of contemporary groups. Approximate reliabilities were calculated for estimated breeding values of the average milk yield in the first three lactations, using the data of daily milk yield of Polish Black-and-White cows in randomly selected herds. Results of approximation were analysed by comparison with corresponding exact reliabilities, obtained from the inverse of the coefficients matrix of the mixed model equations. High correlation of 0.98 for bulls between reliabilities from both methods and very low computer requirements facilitate the implementation of the approximate method into a routine genetic evaluation procedure.