Poor growth is a common finding in patients with organic acidemias. Growth hormone (GH) therapy has been considered in the management of these disorders as a mode to enhance anabolism and lower the high levels of methylmalonic acid. We report two patients with methylmalonic acidemia (mut(o)) and GH deficiency. Both patients had persistently elevated serum concentrations of methylmalonic acid, which failed to respond to conventional therapy. In anticipation of using GH therapy to reduce high methylmalonic acid concentrations, the first patient underwent GH testing utilizing a provocative glucagon stimulation test and was found to be deficient. He was subsequently treated with GH and demonstrated improved growth, but his methylmalonic acid concentrations remain elevated. The second patient was also found to be GH deficient. These findings suggest that GH deficiency may be an etiologic factor in the poor growth seen in patients with organic acidemia.