Studies were performed using 80 children aged 1-3 years with lesions to mental development. Patients were divided into three groups according to the severity and structure of lesions of mental development: those most strongly characterized by delayed speech development (group 1), those with delayed mental development due to organic CNS lesions (group 2), and those with abnormal mental development and high levels of psychopathological symptoms (group 3). There were significant increases (p < 0.001) in blood levels of autoantibodies to nerve growth factor (NGF) in children in each of these groups as compared with a control group (0.75 +/- 0.24 OD units). The levels of anti-NGF autoantibodies increased from group 1 to group 3 (0.95 +/- 0.24 OD units in group 1, 1.13 +/- 0.27 OD units in group 2. and 1.24 +/- 0.4 OD units in group 3). These correlations suggest that the level of anti-NGF autoantibodies can, taken in conjunction with a number of other parameters, be regarded as a potential molecular marker demonstrating abnormal development of the nervous system.