Serum neopterin was measured in patients with acquired 'common variable' hypogammaglobulinaemia (CVH), X-linked agammaglobulinaemia (XLA) and healthy subjects. There was a highly significant increase in neopterin levels in the CVH patients as compared with that of the other groups, particularly in those CVH patients with non-A non-B or granulomatous hepatitis. There was no association between raised neopterin levels and chronic infective lung disease. Since chronic viral infections are associated with raised serum neopterin, this data add some support to a viral aetiology for CVH.