["Participants in trials evaluating preventive interventions such as screening are on average healthier than the general population. To decrease this 'healthy volunteer effect' (HVE) women were randomly invited from population registers to participate in the United Kingdom Collaborative Trial of Ovarian Cancer Screening (UKCTOCS) and not allowed to self refer. This report assesses the extent of the HVE still prevalent in UKCTOCS and considers how certain shortfalls in mortality and incidence can be related to differences in socioeconomic status.", 'Between 2001 and 2005, 202 638 postmenopausal women joined the trial out of 1 243 312 women randomly invited from local health authority registers. The cohort was flagged for deaths and cancer registrations and mean follow up at censoring was 5.55 years for mortality, and 2.58 years for cancer incidence. Overall and cause-specific Standardised Mortality Ratios (SMRs) and Standardised Incidence Ratios (SIRs) were calculated based on national mortality (2005) and cancer incidence (2006) statistics. The Index of Multiple Deprivation (IMD 2007) was used to assess the link between socioeconomic status and mortality/cancer incidence, and differences between the invited and recruited populations.', 'The SMR for all trial participants was 37%. By subgroup, the SMRs were higher for: younger age groups, extremes of BMI distribution and with each increasing year in trial. There was a clear trend between lower socioeconomic status and increased mortality but less pronounced with incidence. While the invited population had higher mean IMD scores (more deprived) than the national average, those who joined the trial were less deprived.'] Does impact on mortality and cancer incidence rates of using random invitation from population register for recruitment to trials?