This investigation of the comparability of writing assessment prompts was conducted in two phases. In an exploratory Phase I, 47 writing prompts administered in the computer-based Test of English as a Foreign Language™ (TOEFL.-CBT) from July through December 1998 were examined. Logistic regression procedures were used to estimate prompt difficulty and gender effects. A panel of experts reviewed selected prompts, and a taxonomy of prompt characteristics was developed and related to prompt difficulty and gender differences. In Phase II, 87 prompts administered from July 1998 through March 2000 were analyzed. All of the prompts used in Phase I, together with 40 new prompts, were analyzed using the larger Phase II database. Recommendations are made for statistical quality control procedures to identify less comparable prompts.