GW bodies (GWBs), or mammalian P bodies, proposed to be involved in messenger RNA storage and/or degradation, have recently been linked to RNA interference and microRNA (miRNA) processing. We report that endogenous let-7 miRNA co-precipitates with the GW182 protein complex. In addition, knockdown of two proteins, Drosha and its protein partner DGCR8, which are vital to the generation of mature miRNA, results in the loss of GWBs. Subsequent introduction of short interference RNA specific to lamin A/C is accompanied by reassembly of GWBs and concurrent knockdown of lamin A/C protein. Taken together, these studies show that miRNAs are crucial components in GWB formation.