We performed clinical, cytogenetic, and molecular analyses on 13 patients (8 females and 5 males, aged 6 months to 13 years) with Wolf-Hirschhorn syndrome due to de novo deletions of chromosome 4p. All patients presented with the typical facial gestalt, microcephaly, and profound mental retardation. Other clinical signs were low birth weight (10/13; 77%), postnatal short stature (8/12; 66%), muscular hypotonia (12/13; 92%), seizures (11/13; 85%), congenital heart defects (4/13; 31%), colobomata of iris (4/12; 33%), genital anomalies (4/13; 31%), deafness (3/13; 23%), and renal anomalies (3/13; 23%). The smallest deletion was a submicroscopic terminal deletion of nearly 2.5 Mb. The largest was a terminal deletion of nearly 30 Mb. Cleft lip/palate, preauricular pits/tags, and congenital heart defects were present only in patients with terminal deletions larger than 10 Mb. The deviations from mean birth weight, birth length, and postnatal head circumference correlated with the size of the deletion. Determining the parental origin of the deletion with microsatellite markers, the maternal allele was missing in three patients and the paternal allele in eight patients. Our observations support the existence of a partial genotype-phenotype correlation in Wolf-Hirschhorn syndrome.