Injection hydrofractures grow in transient linear flow in a low permeability, soft rock. The cumulative injection of water or steam scales with time to the power of 1, not ½ predicted from the theory of linear transient flow. Therefore, either the injection hydrofractures grow with time, or the formation permeability increases with time, or both. A simple mass balance of hydrofracture growth during fluid injection, attributed to Carter, is corrected, extended to the case of variable injection pressure, and presented in a simplified form. The Carter theory predictions are then compared with the growth rate of hydrofracture area calculated independently for two steam injectors. There is remarkable agreement between the modified Carter theory predictions and the independently estimated rates of growth of these two hydrofractures.