It is shown that, when conducting sequential testing of linkage in pedigree samples, (1) type I and type II errors observed are less than expected and (2) the generally accepted method for determining the average sample size, E(N), required for sequential analysis of linkage, underestimates it. A less biased approximation of E(N) is proposed. A wide scattering of actual sample sizes required for completion of sequential analysis is demonstrated, which puts practical use of E(N) into question.