Immunoglobulin serum levels and class III HLA polymorphisms (Bf, C4A, and C4B) have been analyzed in 55 gastric cancer patients (13 having at least a first-degree relative affected by the same tumor) from the Republic of San Marino. This was done to search for possible immunoglobulin deficiencies (in particular IgA), which have been proposed to have a prognostic value in gastric cancer, and to identify possible associations between such a tumor and HLA class III determinants. All subjects had normal Ig levels with the exception of one patient (having the worst prognosis) characterized by a combined IgA, IgG, and IgM deficiency. Normal Ig levels were found in all the examined relatives of the proband. The Bf, C4A, and C4B allele frequencies we found did not differ significantly from those reported for healthy subjects in Italian samples.