Abstract: This paper reports 4 cases of severe combined immunodeficiency disease. Postmortem examination was done in 3 cases. The clinical features of these cases included recurrent infections caused by viruses, bacteria or fungi, absence of detectable thymus on chest X-ray film and dysfunction of T and B lymphocytes. Pathological findings are atrophic thymus, hypoplastic or aplastic lymph nodes and spleen, and only few lymphocytes found on section. Two cases suffered from vaccinia gangrenosa following smallpox vaccination, while one case showed dissemination of living BCG vaccine. It is clear that prophylactic vaccination in these babies may result in severe complications, such as BCG dissemination, vaccinia gangrenosa, vaccine-related paralytic poliomyelitis, etc.