BACKGROUND AND OBJECTIVE: To investigate the effects of knee osteoarthritis (OA) on activities of daily living and motor and functional improvement in stroke patients with hemiparesis. METHODS: This prospective case-controlled study included 60 inpatients (44 women, 16 men; mean age 66.1 +/- 7.6 years, range 51-79 years) with hemiparesis after stroke. Main outcome measures included the Brunnstrom motor recovery stages of the lower extremity, Functional Ambulation Category (FAC), Barthel index, knee pain as assessed by a VAS, and radiographic severity of knee OA based on the Kellgren-Lawrence (K/L) criteria. RESULTS: Between improvements in FAC score and time since stroke, Barthel score, knee pain at rest, K/L radiological grade revealed weak but significant correlations (r: 0.254-0.393) (p < 0.05). Regression analysis revealed that time since stroke, knee pain at rest and radiologically knee ostearthritis had a significant effect on the improvement in ambulation level (r = 0.134, p = 0.001; r = 0.137, p < 0.05; r = -0.007, p < 0.05; respectively). CONCLUSIONS: The present study shows that knee OA had a negative effect on ambulation levels in hemiplegic patients after stroke. Thus, knee pain at rest and knee ostearthritis could be evaluated as a functional outcome factor for the improvement of the ambulation levels within stroke patients.