In 1986-87, the Austin (Texas) Independent School District converted from a full-day prekindergarten program for low-achieving children to a half-day program to serve more limited-English-proficient (LEP) and low-income children (thereby meeting eligibility criteria in conformance with Texas House Bill 72). Half-day instructional time was 60% of that provided in full-day sessions, and with teaching staff increases, three times as many children (1,516) were served as in 1985-86 (494), including more bilingual and English as a Second Language (ESL) classes. Pretest-posttest gains on the Peabody Picture Vocabulary Test-Revised evaluated half-day and full-day program effectiveness, measured against national average gains. Results showed that: (1) half-day program gains in English vocabulary were generally two-thirds those of the full-day program; (2) the greater gains of full-day LEP students occurred at all achievement levels, but the difference appeared to be associated more with teacher experience than with amount of instructional time; (3) for non-LEP students, the difference was greatest for lower achieving students and tended to disappear for students scoring greater than about 80 at pretest; (4) there was little if any difference in the achievement gains of students in morning or afternoon classes; and (5) half-day classes may lead to somewhat lower attendance rates. (LPG)