In an attempt to evaluate subclinical lupus nephropathy, we determined the level of urinary albumin by radioimmunoassay in 27 patients with systemic lupus erythematosus (SLE) who had no evidence of clinical renal involvement. The ratio of urinary albumin to urinary creatinine (Ualb/Ucreat X 100) was significantly higher in patients with SLE (2.56 +/- 2.71) than in normal controls (0.83 +/- 0.72). A significant decrease of urinary albumin level in response to steroid therapy was demonstrated in 5 patients examined repeatedly before and during steroid treatment. In one patient who had two episodes of exacerbations of the disease the urinary albumin level increased parallel with a worsening of the serological data and the appearance of clinical proteinuria followed. It is concluded that many SLE patients even without clinical renal involvement have pathologic albuminuria and the determination of the urinary albumin level by radioimmunoassay in these patients is useful for the management of the disease.