In order to assess the safety and some efficacy aspects of clozapine under UK conditions, 54 in-patients with severe treatment-resistant schizophrenic disorders were evaluated using several scales before and during treatment. Of the 54 evaluated, 26 completed the 26-week study. Of these patients, 20 showed improvement in psychopathology, often to a marked degree, involving both positive and negative symptoms. Some oral-facial extrapyramidal side-effects decreased as well. Two patients developed neutropenia, but recovered on discontinuation of clozapine. The most frequent adverse event was hypersalivation, and five patients suffered from seizures. It is concluded that clozapine is worth considering for the treatment of severe treatment-resistant patients in the UK.