The findings of 23 patients with the del(4p) phenotype are compared systematically. Three patients with a small deletion evident only on analysis of extended chromosomes have a phenotype comparable to 13 patients with a more extensive chromosomal deletion. Two patients with no detectable deletion also fit into the phenotypic spectrum of patients with del(4p), suggesting the same etiology. Five patients with fewer typical and more atypical findings probably represent a heterogeneous group of other syndromes. Numerical analysis of the phenotype allows one to identify patients who most likely have a deletion requiring a more intensive cytogenetics analysis.