Aim: Quality of life (QoL) is increasingly considered an important outcome in health research. We wished to explore the determinants of change in QoL in patients with schizophrenia over the course of a one-year RCT. Methods: Predictors of change in observer-rated QoL (Quality of Life Scale: QLS) were assessed in 363 patients with schizophrenia during the CUtLASS clinical trial. Results: Change in QLS score over the course of a year correlated with change in psychotic and depressive symptoms and treatment adherence. Linear regression showed that improvement in QoL was predicted by reduction in negative and depressive symptoms and improvement in adherence rating. These three change scores together explained 38% of the variance in QLS change. Exploration of the direction of any possible causal effect, using TETRAD, indicated that improved adherence leads to improved QoL, and that change in depression also leads to QoL change. The relationship between QoL and negative symptoms suggests that greater social activity (reflected as better QoL scores) improves negative symptoms. Such a direct relationship between treatment adherence and QoL has not been reported before. Conclusion: Improving adherence to medication would appear to be a key approach to improving measured quality of life in people with schizophrenia.