Sum-of-products variables are integral to testing certain psychological theories (e. g., the theory of reasoned action). A sum-of-products variable is a total score computed by multiplying the scores for corresponding items from two scales and then adding the products. This type of variable is associated with methodological difficulties, however. The purpose of this article is to discuss certain methodological issues associated with the use of sum-of-products variables in health-related research. © 1993 John Wiley & Sons, Inc.