BACKGROUND/PURPOSE This study examined whether an injectable hydrogel could buttress the balloon used in fetal tracheal occlusion, thus preventing its displacement. METHODS Fetal lambs (n = 11) underwent tracheal occlusion through local delivery of a detachable silicone balloon and were divided in 2 groups: group I had no further manipulations, and group II received an intratracheal injection of a rapidly polymerizing hydrogel, cranially to the balloon. Near term, balloon placement was examined, the lung volume-to-body weight ratio (LV:BW) was determined, and tracheal histology was performed. Statistical analysis was by the Fisher's Exact test, with significance set at P <.05. RESULTS Complete tracheal occlusion was achieved in all fetuses intraoperatively. The rate of balloon dislodgement was significantly higher in group I (4 of 7, or 57.1%) than in group II (0 of 4). In group II, balloons were recovered in situ with a column of residual hydrogel reinforcing their cephalad position. Animals in which balloon occlusion was maintained had significantly higher LV:BW, with no evidence of tracheal damage. CONCLUSIONS Intratracheal delivery of a rapidly polymerizing hydrogel cephalad to detachable silicone balloons results in improved fetal tracheal occlusion, with no harmful effects to the trachea. This adjuvant principle may enhance minimally invasive balloon tracheal occlusion for treatment of severe fetal pulmonary hypoplasia.