Experimental models have shown that the reduction in renal mass induces an increase in glomerular filtration rate of the remnant nephrons, leading to proteinuria and glomerular sclerosis. Since the presence of microalbuminuria - increased urinary albumin excretion (UAE) undetectable by routine assays - can be an early sign of this phenomenon, UAE in the normo- and microalbuminuric range was measured in 24 single kidney patients with negative Albustix. Nephrectomy had been performed in 22 cases 1 to 28 years before, mostly because of renal lithiasis. Patients were selected as being normotensive, normoglycemic and free of recurrent urinary infections or stones. On regular diet (mean protein intake 1.2 g/kg/day), UAE mean values were significantly higher in single kidney patients than in 20 controls both in supine position during overnight rest (clinostatism) (37.71 +/- 56.32 vs 2.56 +/- 2.27 micrograms/min, p less than 0.001) and in erect position during moderate physical effort (orthostatism) (67.31 +/- 86.12 vs 4.59 +/- 5.73 micrograms/min, p less than 0.004). Microalbuminuria was observed in 18/24 single kidney patients in clinostatism and 15/24 in orthostatism. A subgroup of 14 patients was also studied on different protein dietetic regimens. After one month on a 0.6 g/kg/day protein containing diet, UAE mean levels significantly decreased in comparison to those found on regular diet (clinostatism: 26.15 +/- 35.93 vs 49.24 +/- 70.29 micrograms/min, p less than 0.02; orthostatism: 31.73 +/- 46.97 vs 68.92 +/- 83.53 micrograms/min, p less than 0.001). One month after a 1.6 g/kg/day protein diet UAE mean values significantly increased (clinostatism 83.99 +/- 88.04 micrograms/min, p less than 0.001; orthostatism: 117.19 +/- 116.12 micrograms/min, p less than 0.001). Our data indicate that microalbuminuria, detectable in the majority of patients with a single kidney, can be modulated by different protein intakes.