Authors studied L.D.H .and Isoenzymes of L.D.H. activities in 254 C.S.F. corresponding to 112 children affected with different diseases. The purpose was to discover its diagnostic utility and possible etiological explanation. Results have been compared with reference values found in a previous study. Highest enzymatic activities of L.D.H. are observed, in decreasing order, in tumours, terminal process, endocranial hipertension plus meningitis, and meningitis. Referring to different types of meningitis, highest enzymatic activity of L.D.H. was found in bacterial type, especially those caused by a germen different from "Neisseria meningitidis". An inversion of isoenzymatic pattern is observed with an increase of L.D.H.-4 and L.D.H.-5 in detriment of L.D.S.-1 and L.D.S.-2, in tumours, endocranial hipertension plus meningitis and in all infections of central nervous system in the initial phase, returning to normal in recuperation phase of disease. A possible enzymatic etiology in different pathological diseases is suggested. In conclusion, it is of interest to determine L.D.H. and its isoenzymes as control of disease but not as differential diagnosis.