We measured cerebrospinal fluid (CSF) and serum beta 2-microglobulin (beta 2-M) in Acquired Immunodeficiency Syndrome (AIDS) patients with or without clinical evidence of central nervous system (CNS) involvement. The CSF beta 2-M level was significantly higher than the serum level in AIDS patients with neurological symptoms, but not in AIDS without neurological symptoms, suggesting an increased shedding of this protein in CSF, as a result of rapid cellular turnover within CNS. CSF beta 2-M level increases both in Human Immunodeficiency Virus type 1 (HIV-1) related and in opportunistic CNS syndromes, confirming that beta 2-M is a non specific marker of CNS involvement in AIDS. Nevertheless, the highest CSF beta 2-M values were observed in patients with severe dementia and autoptic diagnosis of multifocal giant cells encephalitis (MGCE) without other opportunistic diseases. This observation could have important implications for monitoring AIDS dementia complex in AIDS patients. Finally, 5 out of 7 (71%) AIDS patients with cryptococcal meningitis showed a decline in CSF beta 2-M level well related to the decrease of cryptococcal antigen (Crypto-Ag) titres and the clinical remission. This data suggests that CSF beta 2-M determination could be used as a useful test in monitoring efficacy of therapy of CNS pathologies in AIDS patients.