Although some cases of tubular dysfunction (TD) associated with nephrotic syndrome have been described, the incidence and the characteristics of this complication remain unknown. We investigated the presence of TD (renal glycosuria, aminoaciduria, metabolic acidosis with normal anion gap, hypouricaemia, and throughout hypophosphataemia) in 36 patients with nephrotic syndrome. Ten patients (group 1) showed glycosuria at some time during the course of their illness, ranging from 2.5 to 11.2 g/24 h. In addition, seven of them had metabolic acidosis with normal anion gap, five aminoaciduria, and two hypouricaemia. Membranous glomerulonephritis was the most frequent aetiology in group 1 patients (7 of 10). Proteinuria and serum creatinine (SCr) were significantly higher in group 1 patients than in the 26 remaining patients without TD (group 2): 10.2 +/- 3.7 versus 6.7 +/- 2.9 g/24 h (P less than 0.01) and 3.2 +/- 1.9 versus 1.6 +/- 0.9 mg/dl (P less than 0.05) respectively. The appearance of TD coincided with a clear worsening of renal function in most of group 1 patients. In addition, at the end of follow-up, SCr had increased from 3.2 +/- 1.9 to 5.6 +/- 3.3 mg/dl (P less than 0.05) in this group. In contrast, SCr did not show significant changes in group 2 (1.6 +/- 0.9 versus 2.1 +/- 2.2 mg/dl). In conclusion, a significant proportion (27.7%) of patients with nephrotic syndrome present TD data at some moment of their course; the appearance of this complication appears to be a sign of poor prognosis.