The rare bone thickening disease osteopetrosis occurs in various forms, one of which is accompanied by renal tubular acidosis (RTA), and is known as Guibaud-Vainsel of this autosomal recessive syndrome comprise oncreased vone density, growth failure, intracerebral calcification, facial dysmorphism, mental retardation, and conductive hearing impairment. The most common cause is carbonic anhydrase II (CAII) deficiency. Several different loss of finction mutations in CA2, the gene encoding CAII, have been described. To date, there have been no exceptions to the finding of CAII deficiency in patients with coexistent osteopetrosis and RTA. Most often, the RTA is of mixed proximal and distal type, but kindreds are reported in which either distal or proximal TRA predominates.