The frequency of antinuclear antibodies (ANA), the immunoglobulin class of ANA and their specificity for known nuclear antigens were determined in 24-h urine collections from patients with systemic lupus erythematosus (SLE) and progressive systemic sclerosis (PSS). Sixteen % of SLE patients had detectable urine ANA by indirect immunofluorescence using mouse kidney substrate. A higher incidence, 32% of SLE and 28% of PSS patients, had detectable ANA in a titer greater than or equal to 1:16 using HEp-2 cell substrate. IgG ANA was the most frequent immunoglobulin class of antibodies present in the urine; 56% of SLE and 29% of PSS patients with urine ANA had more than one immunoglobulin class of antibodies. Antibodies to Sm, nRNP, SS-A and dsDNA were detected in SLE urine; antibodies to SS-A and centromere were detected in PSS urine. Urine ANA detected on mouse kidney substrate and urine dsDNA antibodies correlated with diffuse proliferative glomerulonephritis in patients with SLE. Sixty-two% of SLE patients with urine ANA had proteinuria. In the remaining SLE patients and in all the PSS patients with urine ANA however, protein excretion was normal. SDS-PAGE revealed heavy and light immunoglobulin molecules in both SLE and PSS patients with urine ANA. The intact immunoglobulin was shown to have ANA activity. ANA present in the urine of SLE and PSS patients with apparently normal renal function may be an early sign of altered glomerular capillary membrane permeability.