Thirty middle- to upper middle-class monolingual Dutch speaking families consisting of at least a mother and a father completed the Infant Form ‘Words and Gestures’ of the Dutch adaptation of the MacArthur Communicative Development Inventory for the same child at 1;1. Considerable inter- and intrafamily variation emerged in how two (or three) different reporters who are all presumably close to the child assess a particular child's communicative abilities. The greater the child's communicative ability, as rated by any one reporter, the more differences tended to emerge between reporters. In order to take into account multiple reporters' assessments of the same child, we propose the use of a Cumulative CDI Score that credits the child with the best score for any item on the CDI as checked by any single reporter. We conclude that single reporter CDI reports may underestimate the child's communicative knowledge.