The validity of the word attack skill tests of the Wisconsin Design for Reading Skill Development is studied at six grade levels by an investigation of the relationship of these to several widely known reading achievement tests. The basic notion is that reading mastery scores derived from the diagnostic tests should be positively related to the achievement scores in a simple manner. The requirements implied by this notion are specified and the procedures for examination of data to determine extent of agreement are described. The results of the analysis indicate that the requirements are fulfilled in all but one case. The main conclusion is that the validity of the word attack subtests is supported by the data. (Author)