The issue of radio frequency interference in Digital Subscriber Line (DSL) networks is of particular concern in urban areas with many strong radio sources nearby. This paper investigates in detail the mechanisms underlying the capacity reduction due to the presence of radio frequency signals from Amplitude Modulated (AM) broadcast stations in an ADSL network. An ADSL system, channel, and radio frequency interference noise were simulated in MATLAB. The capacity reduction due to different Radio Frequency (RF) ingress conditions was studied. Spectral Spreading of the RF interferer, as suggested by Harris et al , is a primary cause of capacity reduction, but only at long loop lengths where the signal to Gaussian noise ratio is very low.