Longitudinal serum immunoglobulin levels were studied in 36 children with selective IgA deficiency during a median follow-up period of 5 years. Twenty-five children were 'sporadic' cases, and 11 were 'familial'. Serum and saliva IgA levels in 23 children remained below 2 mg/l. Eight children with IgA levels above 2 mg/l showed considerable intra-individual variance in serum IgA, but remained IgA deficient. Five children at various ages developed IgA levels above 50 mg/l with detectable secretory IgA in saliva. In most of the children IgG subclass levels were found to be rather high, including at younger ages. There were no obvious differences between 'sporadic' and 'familial' cases except an association between IgD deficiency and serum IgA levels below 2 mg/l, and between serum levels of IgD above 1 IU/ml and of IgA above 2 mg/l, which was found to be significant in the 'sporadic' group but not in the 'familial' group.