Handedness is often the index of cerebral lateralization. It is generally accepted that hand preference can be used as a determinant of hemisphere dominancy. We investigated cognitive functions in relation to hand preference in children to detect probable electrophysiological or mental differences and their clinical correlates between the right or left-handed children. Firstly, Geschwind hand preference test was performed in 4000 children aged between 6-12 years. Then, 60 right and 60 left-handed children from them were chosen to investigate above mentioned points in a greater detail. Their parents hand preferences,exposure of psychological stressors, enuresis, encopresis, language or learning difficulties and tics were investigated in the two experimental groups. Additionally,cognitive function tests, intelligence tests and EEG were performed. No difference was found between the groups in intelligence or cognitive function scores. It was observed that enuresis was double more frequently in the boys and in the left-handed girls were more frequent than boys. Stuttering was more in the left-handed boys than in the girls (p<0.01). There was no significant difference between the right and left-handed groups in frequency of the symptoms encopresis and tics. Hand preference was not correlated with any EEG finding. However, alpha wave amplitudes were significantly higher in nondominant hemisphere than in dominant hemisphere. Background activity of EEG was not related to hand preference because it showed predominantly alpha rythm pattern both in right and left-handed groups. In conclusion, exposure to being impressed by parents or social environment to change child’s hand preference, especially left handedness, after development of cerebral dominance may lead to some mental problems.