Patterns of nonresponse, representing items not reached by test takers or omitted because of difficulty, were addressed for all three age groups (grade 4, grade 8, and grade 12) taking the 1990 National Assessment of Educational Progress (NAEP). The analysis considered nonresponse rates for each item in the seven blocks of items on which reported scaled scores were based. Differences in nonresponse items were also examined for population groups and gender, and the relationship of nonresponse to characteristics of items was also considered. Overall omit rates were modest for grades 4 and 8 in 1990 and the not-reached rates were greatly reduced from those of 1986. Differences between white and minority students were less severe than they initially appeared and gender differences in omit rates were infrequent. Omit rates were high for a subset of open-ended items and the proportion of omitted items was substantial. Differentials for white and minority students in Grade 12 were troubling, and not-reached rates were high for certain blocks of the test. These results suggest the need for routine but focused monitoring and reporting of nonresponse patterns. An appendix presents cognitive complexity classifications. Twelve tables present study findings. (Contains 5 references.) (SLD)