To investigate the influence of arm position on the color and blood flow of the forearm skin, the CIE L*a*b* values, the erythema and melanin indices, and laser-Doppler blood flow values were recorded at three different arm positions (lowered, heart level and raised) in 15 healthy subjects using three kinds of optoelectronic instruments. The values of almost all color parameters, including the melanin index, changed stepwise significantly following the change in the arm position, and blood flow values decreased significantly during arm lowering. These results are most likely due to complex changes in the volume, flow rate and oxygen saturation level of the blood in the superficial vascular plexus, and their influences on each parameter were discussed. To use these instruments effectively for the quantitative evaluation of skin test reactions, a standardized arm position should be kept when the test is carried out in the forearm.