Background: Children's language abilities set the stage for their education, psychosocial development and life chances across the life course. Aims: To compare the efficacy of two preschool language interventions delivered with low dosages in early years settings (EYS): Building Early Sentences Therapy (BEST) and an Adapted Derbyshire Language Scheme (A-DLS). The former is informed by usage-based linguistic theory, the latter by typical language developmental patterns. Methods: We conducted a pre-registered cluster randomized controlled trial in 20 EYS randomized to receive BEST or A-DLS. Children aged 3;05-4;05, who were monolingual, with comprehension and/or production scores [less than or equal to] 16th centile (New Reynell Developmental Language Scales--NRDLS) and no sensorineural hearing impairment, severe visual impairment or learning disability were eligible. A total of 102 children received the intervention. Speech and language therapists delivered interventions with high fidelity in 15-min group sessions twice weekly for 8 weeks. Baseline (T1), outcome (T2), and follow-up (T3) measures were completed blind to the intervention arm. Outcomes were NRDLS comprehension and production standard scores (SS), measures of language structures targeted in the interventions and communicative participation (FOCUS-34). Results: Both interventions were associated with significant change from T1 to T2 and from T1 to T3 in all outcomes. There were no differences between interventions in gains in NRDLS comprehension SS at T2 or T3. BEST produced greater gains in NRDLS production SS between T1-T2 (d = 0.40) and T1-T3 (d = 0.55) and in BEST-targeted sentences (d = 0.77). Children receiving BEST made significantly more progress after intervention (T2-T3) in both comprehension and production. Both interventions were associated with large, clinically significant changes in communicative participation as measured by teacher reports (FOCUS-34). Conclusions: A low-dosage intervention can produce language gains with moderate to large effects. The accelerated progress after the BEST intervention underscores the significant potential of interventions designed with reference to usage-based theory, which precisely manipulates language exposure to promote the specific cognitive mechanisms hypothesized to promote language learning.