The level of beta-aminoisobutyric acid (beta-AIB), a thymine catabolite, has been measured in urine samples of 160 healthy individuals, 28 patients with renal, 27 patients with cardiovascular and 27 patients with hematological diseases and of 36 tumor patients. No significant difference in the prevalence of high excretors of beta-AIB between patients with cancer, renal and cardiovascular diseases and the healthy group was found, whereas all but two patients with hematological diseases were high excretors. Urinary beta-AIB shows a reverse correlation with the hemoglobin level and erythrocyte count in the cases of anemia, and appears to be directly correlated with the leukocyte count and blast cell content in the cases of leukemia, with its amount decreasing two to five-fold with the return of the hematological markers to normal levels after medicinal treatment. Therefore the beta-AIB concentration in urine may be used in combination with hematological indicators in assessing the disease status and in monitoring of the treatment response.