A male infant delivered at term to unrelated parents was found to have multiple dysmorphic facial characteristics, abnormal head shape, anemia, thrombocytopenia, a prominent holosystolic heart murmur with multiple cardiac defects, hypotonia, and was small for his gestational age. Karotype revealed a de novo deletion of the long arm of chromosome 11, del (11)(q23), which has been previously described as Jacobsen syndrome. Recent studies have demonstrated that a folate-sensitive fragile site at 11q, band 23, (11q23) may be responsible for this deletion and possibly other syndromes as well.