Recently Meisler et al. (1980) reported the results of mouse/human somatic cell hybrid studies which indicated that the locus for human uroporphyrinogen I synthase (UPS) (EC 4.3.1.8) maps to chromosome 11. To evaluate further this assignment we have children with a trisomy of the region 11qter. We confirm the results of Meisler et al. (1980) and demonstrate that uroporphyrinogen I synthase activity is increased by a factor of 1.5 in trisomy 11qter. In erythrocytes of one child with a trisomy 11p, the expression of this enzyme was normal.