A patient with congenital hypothyroidism and acute lymphocytic leukaemia was found to be homozygous for a 0.4 kbp deletion in the c-fms proto-oncogene. This was established by studying DNA from the patient's leukaemic cells, from cultured skin fibroblasts of the patient and from normal white blood cells of both parents. The uncertain relevance of this finding to the condition of the patient is discussed.