"Minamata disease" (methylmercury poisoning), originally found in Minamata, Japan 3 decades ago, still poses a medico-socio-legal problem mostly resulting from difficulty in drawing diagnostic demarcation. To examine the efficacy of urinary beta-2-microglobulin (BMG) in identifying patients with Minamata disease, the authors studied 115 patients (63 males and 52 females) who were officially registered for compensation (registered patients); 114 patients (54 males, 60 females) currently undergoing medical evaluations for compensation (unregistered patients); and 82 control subjects (40 males, 42 females) matched for sex and age. There was no significant difference with regard to urinary BMG (corrected by creatinine) among the above three groups, although there was a parallel increase in urinary BMG and neurological scores of male patients which suggested the presence of a dose-effect relationship. Although renal tubular dysfunction manifesting hyper-beta-2-microglobulinuria has been reported in patients with Minamata disease, our findings indicate that excessive excretion of BMG is not a satisfactory diagnostic parameter for Minamata disease.