In newborns, hyperammonemia leads to encephalopathy which is usually characterized by vomiting, hypotonia, lethargy, seizures and coma. Continuous venovenous hemodiafiltration (CVVHDF) is a modality choice to treat acute decompensation in hyperammonemia. Here we report three newborn patients with hypotonia, convulsion and hyperammonaemia. In the first three days of life, their serum ammonia levels were 4609, 1023 and 1949 mu g/ml. They were successfully treated with CVVHDF and serum ammonia levels subsequently decreased to 268, 164 and 65 mu g/ml in the first 24 hours of treatment. The complications were mild hypothermia and anemia.