The diagnosis of myasthenia gravis (MG) can usually be made on the basis of the characteristic clinical history and signs, improvement by the use of anticholinesterase drugs, decremental responses in repetitive nerve stimulations, and assay of anti-acetylcholine receptor (AchR) antibody titers. We, however, have difficulty to make diagnosis of ocular MG patients with mild symptoms because muscular weakness is minimal and ancillary tests are negative. In the present communication, we report clinical usefulness of a hot test to provoke ptosis by warming the eyelid in ocular MG patients with minimal fatigability. Patient 1, a 27-year-old housewife, developed drooping of the right upper eyelid in May 1985. The ptosis was absent in the morning, but became apparent and worsened later. Neurological examination carried out 3 months after the onset revealed mild right ptosis, but fatigability of the levator palpebrae superioris could not be elicited by the provocative procedures such as sustained upward gaze or repeated opening and closing of eyelids. Both Tensilon and cold tests yielded negative responses. Repetitive nerve stimulations produced no decremental responses. Titers of anti-AChR antibody and antistriational antibody were within normal limits. In order to find a possible neuro-muscular blockade, we warmed the right upper eyelid by applying hot water of about 45 degrees C in a vinyl bag for 3 minutes. The hot test worsened the right ptosis and induced mild left ptosis. Tensilon administration reversed the eyelids to the previous position. Patient 2 was a 12-year-old boy with a typical history and clinical signs of ocular MG. His symptoms remitted spontaneously without any medication 3 weeks after the onset.(ABSTRACT TRUNCATED AT 250 WORDS)