We performed an observational study to investigate if plasma 5-hydroxyindole-3-acetic acid (5-HIAA), a derivative end product of serotonin (5-hydroxytryptamine), concentration could be a predictor for deterioration of urinary albumin excretion. The relationship between baseline plasma 5-HIAA concentration and changes in urinary albumin excretion for 24 months was investigated in 162 male patients with type 2 diabetes mellitus. Patients were divided into tertiles according to plasma 5-HIAA concentration. Greater changes in urinary albumin excretion were seen in patients with high plasma 5-HIAA concentration (112.8 +/- 36.2 mg/g creatinine) than in patients with low plasma 5-HIAA concentration (7.6 +/- 8.0 mg/g creatinine, P = .0011) or in patients with intermediate plasma 5-HIAA concentration (25.6 +/- 15.0 mg/g creatinine, P = .0070) after adjustment for baseline values of urinary albumin excretion. A positive correlation was observed between log (plasma 5-HIAA concentration) and changes in urinary albumin excretion (r = 0.314, P < .0001). Multiple regression analysis demonstrated that log (plasma 5-HIAA concentration) (beta = .284, P = .0013) was an independent determinant of changes in urinary albumin excretion. In conclusion, plasma 5-HIAA concentration was positively correlated with changes in urinary albumin excretion, which may indicate causality in diabetic nephropathy in male patients with type 2 diabetes mellitus and high plasma 5-HIAA concentration.