Sir: Patients receiving clozapine must be registered with the Clozaril Patient Monitoring Service (CPMS) for regular haematological monitoring to reduce the risk of agranulocytosis. We report the case of a 55-year-old patient with a 26-year history of paranoid schizophrenia, whose illness had been well-controlled for 4 years with 400 mg clozapine. Unfortunately, the development of chronic lymphocytic leukaemia necessitated withdrawal of clozapine, resulting in a florid relapse of schizophrenia.

A review of the previous test results revealed they were consistently reported as ‘green’ by the CPMS, despite a gradually rising total white cell count from 8 to 20 over the previous 3 years. The total white cell count ranged from 11 to 15 until a recent increase to 20. As this patient's schizophrenia was well controlled on clozapine, this was continued until diagnosis of chronic lymphocytic leukaemia was made at routine review. A subsequent haematological opinion has not suggested any treatment.

There are isolated reports of leukaemia associated with clozapine, but the observed rate is probably the same as the background incidence, with little evidence of a causal relationship. Other haematological abnormalities have been reported in patients taking clozapine, including leucocytosis, lymphopenia, eosinophilia, thrombocytopenia and anaemia (Reference Mendelowitz, Stanton and GersonMendelowitz et al, 1995; Reference Barbui, Campomori and BonatiBarbui et al, 1997). Clinicians should be aware that the CPMS only monitors for a low total white cell and neutrophil count. They should therefore remain alert to the possibility of less common haematological disorders and should not rely entirely on a ‘green’ result from the CPMS.