Antipsychotic medication is a mainstay of treatment for schizophrenia. Risperidone and olanzapine are popular choices among the new generation drugs.To determine the clinical effects, safety and cost effectiveness of risperidone compared with olanzapine for treating schizophrenia.We searched the Cochrane Schizophrenia Group's Register (Sept 2005) which is based on regular searches of, amongst others, BIOSIS, CENTRAL, CINAHL, EMBASE, MEDLINE and PsycINFO. References of all identified studies were inspected for further trials. We also contacted relevant pharmaceutical companies for additional information.We included all clinical randomised trials comparing risperidone with olanzapine for schizophrenia and schizophrenia-like psychoses.We extracted data independently. For homogenous dichotomous data we calculated random effects, relative risk (RR), 95% confidence intervals (CI) and, where appropriate, numbers needed to treat/harm (NNT/H) on an intention-to-treat basis. For continuous data, we calculated weighted mean differences (WMD).We found no difference for the outcome of unchanged or worse in the short term (n=548, 2 RCTs, RR 1.00 CI 0.88 to 1.15). One study favoured olanzapine for the outcome of relapse/rehospitalisation by 12 months (n=279, 1 RCT, RR 2.16 CI 1.31 to 3.54, NNH 7 CI 3 to 25). Most mental state data showed the two drugs to be as effective as each other (n=552, 2 RCTs, RR 'no <20% decrease PANSS by eight weeks' 1.01 CI 0.87 to 1.16). Both drugs commonly cause adverse events: 75% given either drug experience an adverse event; 20% anticholinergic symptoms; both groups experienced insomnia although it was more frequent with risperidone (n=1588, 5 RCTs, RR 1.41 CI 1.15 to 1.72, NNH 15 CI 9 to 41); about 30% experienced sleepiness (n=1713, 6 RCTs, RR 0.92 CI 0.79 to 1.07). People given either drug often experienced some extrapyramidal symptoms (n=893, 3 RCTs, RR 1.18 CI 0.75 to 1.88); 25% of people using risperidone required medication to alleviate these symptoms (n=419, 2 RCTs, RR 1.76 CI 1.25 to 2.48, NNH 8 CI 4 to 25). People allocated to risperidone were less likely to gain weight compared with those given olanzapine and the weight gain was often considerable and of quick onset (n=984, 2 RCTs, RR gain more than 7% of their baseline weight in short term 0.47 CI 0.36 to 0.61, NNH 7 CI 6 to 10). Risperidone participants were less likely to leave the study due to metabolic side effects and weight gain compared with olanzapine (n=667, 1RCT, RR 0.19 CI 0.08 to 0.45). Patients on risperidone were more likely to experience abnormal ejaculation (n=370, 2 RCTs, RR 4.36 CI 1.38 to 13.76, NNH 20 CI 6 to 176). Both drugs are associated with high attrition rates; in the long term consistent findings show that 66% of those allocated risperidone left the study early compared with 56% given olanzapine (n=1440, 5 RCTs, RR 1.17 CI 1.08 to 1.27, NNH 11 CI 7 to 23).We know very little of the effects of these drugs regarding service outcomes, general functioning and behaviours, engagement with services and treatment satisfaction from evaluative studies. There was generally a high rate of attrition in the trials and there appears to be little to differentiate between risperidone and olanzapine except on issues of adverse effects. Both drugs are associated with a reduction in psychotic symptoms but both commonly cause unpleasant adverse effects.