The clinical course of a 17-year-old man with selective IgA deficiency and anaplastic carcinoma in the superior mediastinum is described. High swinging fevers, neutrophil leucocytosis reaching 60 x 10(9)/l, polyarthralgia and skin vasodilatation were unusual manifestations of the tumour. Marked tissue reactions in the neck and polyserositis occurred with a neutrophil granulocyte tissue reaction and progressed to fibrosis. Circulating immune complexes were present and IgE levels were grossly raised. It was not established whether the unusual clinical manifestations were due solely to the tumour or were an abnormal response of an IgA-deficient host. Review of the literature of malignancy in selective IgA deficiency casts doubt upon an increased incidence of neoplasm in this immune deficiency.