Mortality of pups at 8-12 weeks of age was frequently observed in Frisian Water Dogs. Blood parameters and clinical signs of newborns from three litters were monitored. Three pups from two litters displayed strongly reduced levels of immunoglobulins and lymphocytes. These dogs were euthanized after first display of disease. Concurrent clinical and pathological features were consistent with a diagnosis of severe combined immunodeficiency (SCID). Defective V(D)J recombination is one of the causes of SCID in humans and animals. Eight genes involved in V(D)J recombination were investigated by segregation analysis of closely located microsatellite markers and by DNA sequence analysis. A nonsense mutation in the gene coding for V(D)J recombination factor RAG! Was identified in DNA from the cases at a position similar to that of nonsense mutations found in human SCID. It was concluded that SCID due to a mutation of RAG1 led to the high mortality.