We described two cases of idiopathic segmental anhidrosis. Case 1 was a 47-year-old man, who noticed anhidrosis on the right side of face and chest during body heating. Case 2 was a 54-year-old woman, who complained of anhidrosis on the left side of face and upper chest during exercise. Both cases had neither somatic neurological deficit nor autonomic failure except for anhidrosis pupil size and deep tendon reflexes were normal. Reflex sweating to pilocarpine was exaggerated in the anhidrotic areas in both cases, suggesting lesions in the preganglionic sudomotor nerves. Abnormal laboratory finding was elevated serum rheumatoid factor level in only case 2. Segmental anhidrosis was static for 1.5 year and one year, respectively. The previous literature contains similar four cases. Idiopathic segmental anhidrosis may be an abortive form of Ross' syndrome (tonic pupil, hyporeflexia and segmental anhidrosis).