Acidizing treatments can improve well performance. However, treatments for horizontal wells differ significantly from those for vertical wells. To enhance the effectiveness of matrix stimulation in horizontal wells, a simple method of evaluating and optimizing acid treatments, similar to those used for vertical wells, has been developed. A skin factor is calculated explicitly from pressure and flow rate, then used to identify the well response and to understand and control the stimulation process. This analytical solution can be used at the wellsite for field evaluation of acid treatments.