Following perinatal asphyxia and intracranial hemorrhage frequently progressive ventricular dilatation develops in preterm infants. Most common is communicating hydrocephalus due to obliterative arachnoiditis. Ventricular dilatation is reported to affect normal brain development and early therapy is recommended. Cerebrospinal fluid shunting is still accompanied by multiple complications, esp. in preterm infants with a birth-weight below 1,500 g. Seven preterm infants, born between the 27th and 34th gestational week with a birthweight of 910-1,940 g were medically treated for their progressive communicating hydrocephalus. The therapy consisted of intermittant lumbar punctures, medication of acetazolamide and furosemide as well as electrolyte and base replacement. Therapy was started at the 14th-31st postnatal day and lasted from 46 to 149 days. In all children the ventricular dilatation diminished. A steady state of cerebrospinal fluid production and absorption was regained in four children. Due to reoccurrence of ventricular dilatation shunting was performed in three others at the age of more than 3 months and with a weight of 3,620-5,170 g. Thus, medical therapy of hydrocephalus provides time for development of preterm infants, delay of shunting procedures and normalisation of cerebrospinal fluid dynamics.