Late hematoma or seroma and galactocele caused by augmentation mammaplasty have been reported in patients with silicon breast prostheses but are extremely rare in patients injected with polyacrylamide gel (PAAG). In a retrospective survey, the incidence, clinical manifestations, and management of late hematoma, seroma, and galactocele in 28 of 2,610 patients who underwent breast augmentation with PAAG injection were investigated, and 5 typical cases are presented. The diagnostic and managing methods for this complication have been assessed. The incidence of late hematoma or seroma was 0.65% and that of galactocele was 0.35% among patients with PAAG-injected breast augmentations. The clinical onsets of such late PAAG complications were of two types: rapid enlargement in 17 patients and progressive expansion in another 11 patients. Aspiration, ultrasound, and magnetic resonance imaging (MRI) are useful and sensitive tools for diagnosis. Foreign body reaction, PAAG-related tissue necrosis and fibrosis, and granuloma were shown, and the bacterial cultures in all 12 cases were negative. Needle aspiration with pressure dressing has been advocated as a reliable method for small diseases, and surgical exploration with irrigation-vacuum drainage and evacuation with capsulectomy have been considered more effective for recurrent, large, and long-term cases. In conclusion, these late complications rarely present after large-volume injections of PAAG for breast augmentation. The PAAG-related pathologic inflammatory tissue changes are suggested as the pathogenesis for the complication. Trauma and breastfeeding are considered to be stimulating factors.