Chronic management of schizophrenia and schizoaffective disorders is frequently complicated by symptomatic relapse. An open-label, randomized, active-controlled, 2-year trial evaluated 710 patients with schizophrenia or related disorders who were switched from stable treatment with oral risperidone, olanzapine, or conventional neuroleptics to risperidone long-acting injectable (RLAI) or oral quetiapine. Primary effectiveness evaluation was time-to-relapse. Safety evaluations included adverse events (AEs) reported for the duration of the study, Extrapyramidal Symptom Rating Scale (ESRS), clinical laboratory tests, and vital signs. A total of 666 patients (n=329 RLAI, n=337 quetiapine) were evaluable for effectiveness measures. Baseline demographics were similar between treatment groups. Kaplan-Meier estimate of time-to-relapse was significantly longer with RLAI (p<0.0001). Relapse occurred in 16.5% of patients with RLAI and 31.3% with quetiapine. RLAI and quetiapine were both safe and well tolerated. Weight gain affected 7% of patients with RLAI and 6% with quetiapine, with mean end point increases of 1.25±6.61 and 0±6.55 kg, respectively. There were no significant between-group differences in weight gain. ESRS total scores decreased similarly after randomization to either RLAI or quetiapine. Extrapyramidal AEs occurred in 10% of patients with RLAI and 6% with quetiapine. Treatment-emergent potentially prolactin-related AEs were reported in 15 (5%) patients with RLAI and 5 (2%) patients with quetiapine; hyperprolactinemia was reported in 43 (13.1%) patients with RLAI and 5 (1.5%) patients with quetiapine. Somnolence occurred in 2% of patients with RLAI and 11% with quetiapine. To our knowledge, this is the first report of a randomized clinical trial directly comparing relapse prevention with a second-generation long-acting injectable antipsychotic and oral therapy. Time-to-relapse in stable patients with schizophrenia or schizoaffective disorder was significantly longer in patients randomized to RLAI compared with those randomized to oral quetiapine. Both antipsychotics were generally well tolerated.