OBJECTIVE: A lateral tunnel-total cavopulmonary connection (LT-TCPC) using a right atrial (RA) free wall is the first choice of treatment for patients with a small body weight in this institute. Whether the growth of the LT is appropriate or not according to the growth of the body remains controversial. In addition, the optimal initial diameter of an LT is unknown. The purpose of this study was to verify the growth of the LT. METHODS: Ninety-one patients of a total of 267 TCPC cases underwent an LT-TCPC at less than 5 years of age in this institute between March 1991 and June 2008. The data on 47 of the 91 patients, which were available to investigate the LT growth, were retrospectively reviewed. The mean age at LT-TCPC was 37+/-11 months (16-57 months). The mean body weight at TCPC was 12.4+/-2.4 kg (7.6-20.0 kg). The initial LT diameter was determined with Hegar's sizer of the estimated half-pulmonary arterial (PA) diameter, which is a diameter that results in half of the dimension of the normal pulmonary valve. The measured maximum LT diameter (mm) divided by the estimated half-PA diameter (mm) was considered as the LT index. The size of the LT was evaluated using either echocardiography or angiography. RESULTS: The mean follow-up period was 7.4+/-3.5 years (1.6-13.5 years). The LT index was initially 2.0+/-0.7 (1.3-4.5), 2.0+/-0.4 (1.3-3.2) at 1 year after operation, 2.1+/-0.5 (1.5-3.2) at 5 years after operation, 1.9+/-0.4 (1.5-2.8) at 10 years after operation and 2.1+/-0.5 (1.6-2.5) at 13 years after operation, respectively. CONCLUSIONS: LT growth suitable for the body growth can be expected. Although there was some variation in the initial LT diameter, the LT index tended to converge at 2.0 with growth.