Five days after birth of a viable Fell pony filly, yellow watery diarrhoea appeared without any signs of systemic disease. Four days later the diarrhoea ceased. On 11th day, the animal showed apathy, and a few days later, the foal was very lethargic, suffered from muscular weakness and severe watery diarrhoea that reappeared. The illness did not respond to therapy. At the age of 21 days the filly spontaneously died under symptoms of intestinal colic and pneumonia. Haematological examinations revealed lower numbers of erythrocytes as well as non-selective lymphopenia. Phagocytic activity was slightly increased, lymphocyte activity was inhibited. Histopathology showed severe alteration of the lymphatic organs. T and B lymphocytes and antigen-presenting macrophages were not arranged in characteristic areas, and the quantity of these cells was lower than would be expected. Histopathological changes in lymphatic organs resembled those described in the literature as severe combined immunodeficiency.