The responses of an anaerobic fixed-film reactor (AFFR) to hydraulic shock loadings were studied. The AFFR was started up with a synthetic wastewater of 3000 mg COD/l at 5.00 d hydraulic retention time (HRT), achieving 98.1% COD removal efficiency. When stable operation was attained, the HRT was sporadically adjusted to 2.50, 1.25, 1.00 and 0.50 d to simulate two, four, five and 10 times hydraulic shock loadings, respectively, while the COD loading was maintained constant. Under two, four and five times hydraulic shock loadings, the COD removal efficiency was temporarily reduced to between 84.4 and 88.3%, and the pH of the treated effluent and the biogas production were also affected. The AFFR could recover from the temporary inhibition due to these shock loadings and resumed normal operation within eight days. On the other hand, under 10 times hydraulic shock loading, the treatment performance deteriorated drastically. Volatile fatty acids (VFAs) accumulated in the AFFR liquor, resulting in reactor souring and failure. When the HRT was returned to 5.00 d, the biofilter could be recovered within a few days. The ability of the AFFR to recover from critical hydraulic shock loadings and system failure was attributed to the immobilized-biofilm design, which enabled the temporarily inhibited biofilms to be retained in the AFFR and regain activity when favourable conditions were restored.