The brassinosteroid (BR) biosynthetic pathway, and the sterol pathway which is prerequisite to the BR pathway, are rapidly being characterized because of the availability of a large number of characteristic dwarf mutants in Arabidopsis. Here we show that the Arabidopsis dwarf5 mutants are disrupted in a sterol Δ⁷ reduction step. dwf5 plants display the characteristic dwarf phenotype typical of other BR mutants. This phenotype includes small, round, dark‐green leaves, and short stems, pedicels, and petioles. Metabolite tracing with ¹³C‐labeled precursors in dwf5 verified a deficiency in a sterol Δ⁷ reductase activity. All six independent alleles contain loss‐of‐function mutations in the sterol Δ⁷ reductase gene. These include a putative mRNA instability mutation in dwf5‐1, 3′ and 5′ splice‐site mutations in dwf5‐2 and dwf5‐6, respectively, premature stop codons in dwf5‐3 (R400Z) and dwf5‐5 (R409Z), and a mis‐sense mutation in dwf5‐4 (D257N). The dwf5 plant could be restored to wild type by ectopic overexpression of the wild‐type copy of the gene. Both the Arabidopsis dwf5 phenotype and the human Smith–Lemli–Opitz syndrome are caused by loss‐of‐function mutations in a sterol Δ⁷ reductase gene, indicating that it is required for the proper growth and development of these two organisms.