We investigated the role of bilingual parents' language proficiency in their reports of their children's vocabulary size. Sixty-four Spanish-English bilingual mothers whose L1 was Spanish reported their bilingual children's English and Spanish vocabularies and 37 monolingual L1 English-speaking mothers reported their monolingual children's English vocabularies--both using the MacArthur-Bates Communicative Development inventories. Expressive vocabularies were also assessed with a standardized, examiner-administered test. Spanish L1 mothers with limited English proficiency provided lower estimates of their children's English vocabulary than those with higher English proficiency--relative to the examiner-administered test score. Despite this effect of language proficiency on parent reports, the size of the observed difference in English vocabulary between monolingual children and bilingual children was not significantly different when estimated with a parent-report instrument than when estimated with an examiner-administered test.