A system and method of determining the geographical location of a femtocell (640A) of an asynchronous cellular wireless communication system (600) are provided. Timing data is obtained from communication signals received by a first group of mobile communication units (450) from a femtocell (640A), and from at least two macro cells (410A, 420A, 670A). A difference (D12) in path distance from two macro cells to the femtocell (640A) is estimated using the timing data. A correction is made to remove timing offsets of the base stations of the macro cells. This provides an estimate of the geographical location of the femtocell (640A), when combined with either a similar estimate derived from signals received from a third macrocell, or an estimate derived from signal propagation data.