Systematic studies of cognitive development in individuals with congenital giant interhemispheric cysts (GIC) without neurosurgical intervention are rare. However, data from such studies are of great importance in deciding whether or not to perform cerebral shunting in otherwise asymptomatic patients. In this study, psychomotor function was examined in 13 children (three females, 10 males; mean age at follow-up: females 1 year 6 months, males 3 years 2 months; age range: females 13 months to 4.2 years, males 10 months to 10 years) all diagnosed with GIC during the neonatal period after indicative foetal ultrasound. Eight patients received neurosurgical treatment due to increased intracerebral pressure during the first year of life. Psychomotor development was evaluated using Griffith's Psychomotor Developmental Scales, the Sniders–Oomen Non-verbal Intelligence Scale for Young Children and the Kaufmann Assessment Battery for Children. The majority of patients showed normal intelligence, irrespective of the presence or absence of neurosurgical intervention. Evidence was found that the cognitive outcome might be related to an association of GIC with agenesis of the corpus callosum or additional congenital cerebral malformations, such as gyration anomalies. Main postoperative complications were chronic subdural effusions with spastic cerebral (hemi)-paresis; were present in three patients. In view of the risk of severe postoperative neurological complications and a comparable psychomotor development in patients without neurosurgical treatment, recommendations for early neurosurgical intervention in otherwise asymptomatic patients with a foetal or neonatal diagnosis of GIC should be viewed with caution.