In a 3-year-old girl, who was admitted to our hospital with marked splenomegaly, a swollen abdomen and progressive loss of weight, the difficulty in establishing the correct diagnosis, namely a very rare generalized infection with Mycobacterium avium (serum type III), and the lengthy course of the disease are demonstrated. Norcardiosis, sarcoidosis and BCG-granulomatosis were excluded. From the spleen, lymphatic nodes, gastric juice, stools and urine of the child, who had not been vaccinated with BCG vaccine, masses of acid-fast bacilli were isolated. The generalisation of the disease was promoted by an immunological deficiency. At autopsy an atrophic thymus was discovered. The extensive clinical and immunological investigations pointed to a partial deficiency of the cell-mediated immune systeme. The patient died 8 months after admission.