This study presents an experiment that explores the patterns of answers to yes-no truth-functional questions in English and Korean. The answering patterns are examined from 12 Korean-English bilingual children and 10 Korean-monolingual children. Four types of sentences in relation to given situations (Wason in "Br J Psychol" 52:133-142, 1961) were provided as questions such as true affirmative (TA), true negative (TN), false affirmative (FA), and false negative (FN). The bilingual children's answers were observed in separate language settings, English and Korean. The results by the bilingual in the Korean setting were compared with those by the monolinguals. The results show that bilingual children can process two systems rather successfully by providing correct responses to the given questions. But difficulty patterns, measured from error rates in each setting, are found different in two languages. The bilinguals' difficulty patterns in English and Korean, however, show deviation from monolinguals' difficulty patterns suggested in previous studies (Wason in "Br J Psychol" 52:133-142, 1961, Akiyama in "Dev Psychol" 20:219-228, 1984, Kim in "Dev Psychol" 21(3):462-472, 1985, Choi in "Dev Psychol" 29(3):407-420, 1991). The present work also shows that negatives are not uniformly reported with more errors than affirmatives when the truth condition and the answering system are further involved. All in all, the current study suggests that bilingual children have two separate processing systems for yes-no truth-functional questions. However, the two systems cannot be understood as a simple coexistence of two monolingual systems. Interaction of the two competing linguistic systems is discussed further.