OBJECTIVE: To evaluate the effects of an aquatic physiotherapy method (Halliwick-Therapy) upon mobility in the post-acute phase of stroke rehabilitation. DESIGN: Randomized controlled trial. SETTING: Hospital for neurological rehabilitation. PARTICIPANTS: Adult patients after first-ever stroke in post-acute inpatient rehabilitation at least two weeks after the onset of stroke (n = 30). INTERVENTIONS: In the Halliwick-Therapy group (n = 14) the treatment over a period of two weeks included 45 minutes of aquatic therapy three times per week and a conventional physiotherapeutic treatment twice a week. Subjects in the control group (n = 16) received conventional physiotherapeutic treatment over a period of two weeks five times per week. OUTCOME MEASURES: The primary outcome variable was postural stability (Berg Balance Scale). Secondary outcome variables were functional reach, functional gait ability and basic functional mobility. RESULTS: Compared to the control group, significantly more subjects in the Halliwick-Therapy group (83.3% versus 46.7%) attained significant improvement of the Berg Balance Scale (P < 0.05). Improvement of the functional gait ability was significantly higher in the Halliwick-Therapy group (mean (SD) 1.25(0.86)) than in the control group (mean (SD) 0.73 (0.70)) (P < 0.1). The mean differences of improvements in functional reach and basic functional mobility were not statistically significant between groups. CONCLUSIONS: This study indicates that Halliwick-Therapy is safe and well tolerated in stroke patients in post-acute rehabilitation and has positive effects upon some aspects of mobility.