Progressive multifocal leucoencephalopathy (PML) is caused by a papovavirus but serum antibody titres are generally considered unhelpful in clinical diagnosis because antibodies to the commonest causal agent (JC virus) are frequently found in normal adults. There is little published information about CSF titres but usually they have not been useful. Two cases of PML, confirmed by autopsy, are described where CSF antibody to JC virus was measured. In one case the JC antibody titre was significantly higher in the CSF than the serum and we suggest that this finding is diagnostically useful. In this case there was a transient stabilization of the disease following treatment with cytarabine with a change in antibody titres suggestive of reduced viral replication in the central nervous system and a host response to the infection. In the other case, which was untreated, rising serum antibody levels indicated active infection with a host response.