While mesoscale convective systems (MCSs) are major contributors to severe weather and precipitation in the central United States, details of their evolution are still not fully understood. In particular, recent observational and modeling studies have indicated that many MCSs tend to exhibit an asymmetric precipitation pattern during the mature-to-later stages of their life cycles. However, to what extent and how this evolution occurs in all cases is still a matter of conjecture. The recently established WSR-88D network has afforded the opportunity to investigate the evolution of MCSs in greater detail than has been possible in the past. As a preliminary effort in this direction, 13 MCSs having leading line-trailing stratiform structures that occurred over the lower Midwest during May and June 1995 have been studied. It is found that indeed there does appear to be an evolution of such MCSs toward asymmetry later in their life cycles; however, the nature of the evolution is complex. The trailing stratiform region develops an asymmetric pattern (stratiform precipitation located toward the northern edge of the MCS) well before the system reaches maturity. The convective line also evolves to an asymmetric pattern (the most intense convective cells concentrated along the southern end of the line), but the appearance of asymmetry occurs several hours later than it does in the stratiform component. The limited sample size, however, precludes definitive conclusions about the significance of this timing difference. Contributions to asymmetry by hydrometeor advection in the along-line component of the flow aloft have also been inferred from sounding data. Results indicate that the system-relative, along-line flow in the pre-MCS environment is comparable to and often greater than the cross-line flow. This evidence suggests that both alongline and cross-line, system-relative flows, along with other processes (e.g., Coriolis effects), may contribute to an evolution toward asymmetry.