Patients with Ehlers Danlos Syndrome type VI (EDS VI) are biochemically characterized by a deficiency of lysyl hydroxylase (LH), an enzyme that hydroxylates lysine residues required in the formation of stable crosslinks in collagen biosynthesis. Recently, in 19% of 35 EDS VI families, a duplication of seven exons in the LH gene has been identified as a common cause of EDS VI. We have observed that in fibroblasts from patients with this duplication mutation, administration of hydralazine, an iron-chelating agent, and ascorbate, a cofactor for LH activity, stimulates LH activity and its mRNA significantly more than in other EDS VI patients who do not have this duplication. Administration of these reagents, either singly or in combination, to fibroblasts from five patients homozygous for the duplication stimulated the low basal level of LH activity (<20% of normal) by five- to sixfold (hydralazine +/- ascorbate) and by twofold (ascorbate alone) at 72 h. This paralleled the increase in the steady-state level of mRNA for LH measured in similarly treated fibroblasts from four of these patients. In contrast, the activity of LH in fibroblasts from six other EDS VI patients and the mRNA from four of these patients who did not have the duplication were increased only three- to fourfold by hydralazine +/- ascorbate. The mechanism for the preferential stimulation of LH activity and mRNA by hydralazine in the EDS VI cells carrying the duplication is unknown, but it could be attributed to the presence of, for example, an enhancer sequence within the duplicated region of the LH gene.