Fifty-seven Tanzanian children, 6-25 months, hospitalized with acute diarrhoea were grouped according to whether there was clinical evidence of systemic infection (SI) (n = 35) or not (n = 22). Serum acute phase proteins were measured in samples taken within 48 h of admission. Means for C-reactive protein (CRP) and serum amyloid A (SAA) were significantly higher in children with SI compared to those without (geometric means (95% CI); CRP, mg/l: 22.1 (13.6-35.5) vs. 7.4 (4.4-12.4); SAA, mg/l: 12.2 (6.8-22.1) vs. 4.9 (2.5-9.7)). Levels of alpha1-acid glycoprotein were similar in both groups (1.16 g/l (0.95-1.43) vs. 1.04 (0.83-1.29), respectively). CRP > or =30 mg/l had a positive predictive value of 95%, and specificity of 96% for correctly identifying SI, but a low sensitivity (51%) and negative predictive value (55%). Clinical outcome of diarrhoea was worse in children with SI: more needed intravenous fluids (23% vs. 5%), the duration of diarrhoea was longer (59.4 vs. 34.2 h) and mortality was higher (6% vs. 0%). APPs were not found to be useful markers of systemic illness in acute diarrhoea in this population.