The traditional melt index test (ASTM D 1238) is widely used in the plastics industry to characterize polymer processability. The data from this measurement must be interpreted cautiously, however, because polymers are usually processed under conditions that are far removed from those of the melt index test. In this study, melt index (MI) values from a series of polystyrene, polypropylene, linear low-density polyethylenes (butene and octene copolymers), and high-density polyethylenes were measured and related to molecular weight distributions of these materials. It was found that a simple relationship between 1/MI versus M̄xw (where x = 3.4–3.7) was followed for the linear polymers with similar polydispersities. For branched polymers, the best correlation was that of -ln(MI) versus ln(M̄v). A general relation for shear modifiable polymers like polyethylenes cannot be obtained unless the rheological state of the material can also be defined.