Aquifer parameter determination in practical hydrogeological studies exposes one of the most significant steps in groundwater resources assessment and management. Successful parameter estimation necessitates both a reliable methodology and field data. Different analytical solutions in the forms of type curves depend on well and aquifer configuration such as porous or fractured media; confined, unconfined, and leaky conditions; and small-and large-diameter wells. Most often Cooper-Jacob (CJ) straight line is used for aquifer parameter estimations from late time-drawdown data in cases of confined and unconfined aquifers. This straight-line method is attractive because of its simplicity and straightforward calculations without tedious and dubious type-curve matching procedure. The warranty of the CJ method application depends on the fact that the late time-drawdown measurements fall along the final portion of the Theis type curve. The main purpose of this paper is to present a general type straight-line model to determine aquifer properties of a confined leaky (semiconfined) aquifer by using the pumping test results. The CJ method is applicable in cases in which the straight-line slope on dimensionless semilogarithmic paper is equal to 2.3 only. DOI: 10.1061/(ASCE)IR.1943-4774.0000521. (C) 2012 American Society of Civil Engineers.