OBJECTIVES: When conducting systematic reviews of randomised controlled trials (RCTs), an important step is to critically assess the quality of included studies. As emphasis on sound trial design has increased over the years, we sought to establish how quality of studies included in systematic reviews is associated with the publication year of those studies. METHODS: We analysed the studies included in 4 large, clinical systematic reviews of randomised controlled trials conducted by Heron in the last 6 months. These covered 4 disease areas: oncology, hyperlipidaemia, anaesthesia, and serious bacterial infection. The data extracted included a critical appraisal of study quality based on means of quality scales. These were the Jadad scale, which assesses randomisation, blinding, patient attrition and reporting quality and gives a score from 1 (low) to 5 (high), and the allocation concealment grade, which assess whether allocation to treatment was adequately concealed. We analysed trends in both Jadad score and allocation concealment grade by year of publication, with stratification by disease area. RESULTS: A total of 291 trials were included in the analyses from the 4 reviews. Most of these were carried out between 1981 – 2008. The average overall Jadad score was 2.3, suggesting low overall trial quality. Jadad score increased from a mean of 1.4 over the period 1981-1986 to 2.9 over the period 2001-2006. Furthermore there was an increase in the percentage of high quality (Jadad score 4-5) trials – from 6% in 1981-1986 to 36% in 2001-2006. Only 3% of RCTs from the period 1981-1990 recorded adequate allocation concealment; this increased to 26% over the period 1996-2006. CONCLUSIONS: There was an observable increase in trial quality over the period analysed. However, the mean Jadad score, and the proportion of trials with adequate concealment of allocation remained low, even by 2006, indicating the need for continued attention to study quality.