Short-term correction of metabolic acidosis in normal and uremic subjects has been shown to decrease protein degradation, but the long-term effects of better correction of acidosis on nutrition in ESRF are unknown. The aim of this study was to assess the possible benefits, in the nutritional state and morbidity, of improved correction of acidosis in the first year of treatment with continuous ambulatory peritoneal dialysis (CAPD). Two hundred consecutive new CAPD patients were randomized, in a single-blind fashion, to receive a high (HA; lactate 40 mmol/liter) or low (LA; lactate 35 mmol/liter) alkali dialysate for one year. Calcium carbonate and sodium bicarbonate were also used to correct acidosis in the HA group. At one year, the venous serum bicarbonate and arterial pH were 7.44 +/- 0.004 and 27.2 +/- 0.3 mmol/liter in the HA group, and 23.0 +/- 0.3 mmol/liter and 7.4 +/- 0.004 in the LA group (P < 0.001). Dialysis dose, at one year or at the point of leaving the study (HA 8.0 +/- 0.1 liters/day vs. LA 8.5 +/- 0.3 liters/day) was not significantly different (P = 0.18). At one year, the increase in body weight in the HA group (6.1 +/- 0.66 kg) was higher than in the LA group (3.71 +/- 0.56 kg, P < 0.05). The increase in midarm circumference in the HA patients (1.26 +/- 0.16 cm) was significantly higher than the increase in the LA patients (0.61 +/- 0.16 cm, P < 0.05). The increase in triceps skinfold thickness were not significantly different (HA 2.5 +/- 0.41 mm vs. LA 1.24 +/- 0.38 mm, P = 0.1). Serum albumin was 37.8 +/- 0.4 g/dl at one year in the HA group, and 38.2 +/- 0.5 g/dl in the LA group (NS). Dietary protein intake at one year (HA 0.9 +/- 0.2 g/kg/day vs. LA 1.0 +/- 0.1 g/kg/day) was not significantly different. There were fewer hospital admissions in the HA group (1.13 +/- 0.16 per patient per year) compared to the LA group (1.71 +/- 0.22 per patient per year, P < 0.05). The HA patients spent less days in hospital per year than the LA patients (16.4 +/- 1.4 days/year vs. 21.2 +/- 1.9 days/year; P < 0.05). It is concluded that better correction of metabolic acidosis leads to greater increases in body weight and midarm circumference, but not triceps skinfold thickness, in the first year of CAPD. The improvement in morbidity, in terms of number of admissions and days in hospital per year, may be associated with the improvement in nutritional state.