The purpose of this study was to investigate validity and reliability of measuring the quadriceps femoris muscle strength on the affected side in hemiplegic patients with a hand-held dynamometer(HHD) to prepare for a multi-center cooperative study. Measurements of the known weights(8.972 and 18.665 kg) with HHD were 8.98 +/- 0.00 kg and 18.57 +/- 0.00 kg, respectively, and measurements with HHD were almost consistent with those with KIN-COM (gold standard), because the Pearson's product-moment correlation coefficient was 0.99 (P < 0.001). There were significant differences in measurements of the quadriceps femoris muscle strength on the affected side in a hemiplegic patient between just anterior to the face of the ankle and 7 cm proximal to the ankle, and on a chair with or without a backrest (t-test, P < 0.05), but no significant difference was found between the 90 degrees and 60 degrees flexed position of the knee (t-test, 0.1 > P > 0.05). There were also no significant differences in muscle strength measured by three different examiners and on four trials, but significant differences were found among three patients (General Linear Model, P < 0.001). Therefore, by using the standardized measuring procedure, the HHD is reliable to measure strength of the quadriceps femoris muscle on the affected side in hemiplegic patients.