A double-blind, randomized, multi-center, parallel-group study was conducted in Finland to compare the efficacy and safety of risperidone with zuclopenthixol in patients with acute exacerbations of schizophrenia or schizophreniform disorder. Ninety-eight patients were randomly assigned to treatment with risperidone (n = 48) or zuclopenthixol (n = 50), in variable doses, for 6 weeks. The mean daily doses of risperidone and zuclopenthixol at the end of the trial were 8 mg and 38 mg respectively. Efficacy was assessed throughout by the Positive and Negative Syndrome Scale for schizophrenia and Clinical Global Impression. Safety assessments included the Extrapyramidal Symptom Rating Scale, UKU Side-Effect Rating Scale, vital signs, body weight and laboratory screening. The results indicate that risperidone is at least as effective as zuclopenthixol for the treatment of acute schizophrenic episodes, with a trend towards greater improvement in the overall severity of symptoms. The onset of action was significantly shorter with risperidone than with zuclopenthixol. Although the general tolerability of the two drugs was comparable, fewer patients experienced extrapyramidal symptoms with risperidone, so that significantly fewer risperidone-treated patients required antiparkinsonian medication.