We used 18F-fluorodeoxyglucose and PET to study a 23-year-old woman with alternating hemichorea and primary antiphospholipid syndrome. There were three PET studies: (1) during an episode of right hemichorea, (2) during an asymptomatic period 6 months later, and (3) during an episode of left hemichorea occurring 2 months after that. In each study, we calculated normalized regional glucose metabolism for the caudate and lentiform nuclei and compared these values with those calculated in 12 normal volunteer subjects (mean age, 35.3 +/- 9.0). The following results were obtained: (1) during right hemichorea, left lentiform metabolism was increased by 19% (> 3 SD); (2) during the asymptomatic period, right caudate metabolism was increased by 20% (> 3 SD), and right lentiform and left striatal metabolism were normal; and (3) during left hemichorea, right caudate and lentiform metabolism were both elevated by 33% (> 3 SD). During this episode, left caudate metabolism was elevated by 20% (> 2 SD); left lentiform metabolism was normal. These results suggest that hemichorea in primary antiphospholipid syndrome may be associated with contralateral striatal hypermetabolism that may also be present during asymptomatic periods.