The clinical features of patients with schizophrenia who respond to electroconvulsive therapy (ECT) are uncertain. There is also little information on the nature of symptomatic improvement with this treatment. Using a standardized protocol, 253 patients with treatment-resistant schizophrenia (TRS) were treated with combination ECT and flupenthixol, and 138 (54.5%) patients met response criteria. Independent of gender, duration of the current episode and severity of baseline negative symptoms were predictive of outcome. Duration of illness had weak relations with outcome and only among females. There were marked sex differences in other clinical features and symptoms associated with response. In contrast, no gender differences were observed in the nature of symptomatic improvement. Treatment resulted in marked improvement in specific positive symptoms, with an intermediate effect on affective symptoms, and no effect or worsening of specific negative symptoms. The findings challenge recommendations that long duration of illness or absence of affective symptoms portends poor response to ECT in patients with TRS. Gender may play a critical role in determining the features of the illness that predict outcome.