Objective: To examine the effects of trunk exercises performed using the physio ball as against the plinth, on trunk control and functional balance in patients with acute stroke.Design: An observer-blinded pilot randomized controlled trial.Subjects: Thirty patients with acute stroke (mean post-stroke duration 12 (95% confidence interval (CI) 2–34) days) who had the first onset of unilateral haemorrhagic or ischaemic lesion and an independent ability to sit for 30 seconds.Setting: Inpatient stroke rehabilitation centre.Interventions: The experimental group performed task-specific trunk exercises on an unstable surface (physio ball) while the control group performed them on a stable surface (plinth). In addition to regular acute physiotherapy, both the groups underwent 1 hour of trunk exercises a day, four days a week for three weeks.Main measures: Trunk Impairment Scale and Brunel Balance Assessment.Results: The difference between the baseline characteristics of the patients belonging to both groups was not statistically significant. Post-intervention, both the groups improved on trunk control and functional balance but the experimental group improved more significantly than the control group (change scores of between-group comparison for the total Trunk Impairment Scale 3.06 (1.43), dynamic sitting balance 1.47 (1.36) and coordination 1.3 (0.67) subscales of Trunk Impairment Scale; the total Brunel Balance Assessment 1.8 (1.4) and stepping 1.87 (1.6) component of Brunel Balance Assessment). The level of significance was set at P < 0.05.Conclusions: The trunk exercises performed on the physio ball are more effective than those performed on the plinth in improving both trunk control and functional balance in acute stroke patients, suggesting a task-specific effect and also a carry-over effect.