Controversy remains about the growth of the pulmonary arteries following a bidirectional superior cavopulmonary anastomosis in children with complex cyanotic congenital cardiac malformations. This is partially due to the morphological heterogeneity of the patients, and partially due to methodological differences in series published so far. It is further complicated by the variable use, in different centres, of additional sources of pulmonary blood flow. We believe that the fate of these arteries preoperatively is significantly influenced by the amount of pulmonary blood flow and the initial size of the arteries. Separate assessment of the pulmonary arterial development postoperatively is recommended for those who, initially, had relatively small as opposed to larger than normal pulmonary arteries. Measurement of the diameters of both pulmonary arteries just prior to their first point of branching, together with the use of Z-score evaluation rather than the Nakata index, is discussed. It remains to be established whether, over time, the bidirectional cavopulmonary anastomosis is effective in developing adequately the pulmonary arteries in preparation for an ultimate total cavopulmonary connection, or even as isolated long-term palliation. A prospective, multi-institutional study involving sequential non-invasive assessment of pulmonary arterial development (using, for example, magnetic resonance imaging) is required to solve the outstanding problems.