Although many common problem-solving errors within the realm of school mathematics have been previously identified, a compilation of such errors is not readily available within learning disabilities textbooks, mathematics education texts, or teacher's manuals for school mathematics texts. Using data on error frequencies drawn from both the Fourth National Assessment of Educational Progress and a study of the problem-solving performances of 431 fifth- and sixth-grade students from southern California, error patterns and their probable sources for traditional one-step word problems employing whole numbers can be described. Problem-solving errors are divided into four broad categories: (1) computational errors; (2) operational errors; (3) non-attempt errors; and (4) miscellaneous errors. Teacher knowledge of error patterns in problem solving promotes positive outcomes, yet teachers are often lacking in knowledge of error pattern analyses, particularly teachers of students with learning disabilities. The findings indicate not only that appropriate student-error analysis leads to accurate feedback on the source of student errors, but also that an understanding of error patterns can reveal underlying conceptual misunderstandings and/or lack of proper problem-solving strategies on the part of the students; such findings can ultimately become the focus for further instructional planning. (JJK)