This study examined the isoenzymatic pattern of LDH in the cerebrospinal fluid (CSF) as well as the ratio between the five fractions of LDH among patients with various brain tumours, carcinomatous meningitis and control groups. LDH 1/LDH 2 <1 was found significant for carcinomatous meningitis (p < 0.001) and brain metastases (p < 0.001). LDH 1/LDH 2 ratio was found to be significantly lower in carcinomatous meningitis than in brain metastases (p < 0.05). No LDH 1/LDH 2 ratios smaller than 1 were found in the other groups. The LDH 1/LDH 2 ratio smaller than 1 was found in the early stage of carcinomatous meningitis without other evidences of the involvement of the leptomeninges. Examination of LDH 1/LDH 2 can be found as an adjunctive method to identify brain metastases and carcinomatous meningitis at the initial stage.