During recent years, a decrease has been noted in the rate of transition of ultra-high risk (UHR) clients to a psychotic disorder. Although important to the concept of the at-risk mental state, the reasons for this decline remain largely unknown. We investigated the possibility of a 'dilution effect' in contributing to the decline, i.e. if later UHR cohorts present with less severe clinical intake characteristics than earlier cohorts.Firstly, clinical intake characteristics of a large UHR sample (n = 397) were compared across baseline year epochs (1995-2006). Characteristics showing significant differences were included in a Cox-regression to examine if they could explain the decline in transition rates. Secondly, because later cohorts show lower transition rates, 'more stringent' UHR-criteria were retrospectively applied to these cohorts (post-2000, n = 219), investigating if this resulted in a higher transition rate.Results indicated that earlier cohorts presented with (1) a larger array of attenuated psychotic symptoms, (2) higher ratings on conceptual disorganization (formal thought disorder) and (3) a higher proportion of individuals with trait risk factor (all P. <. .001). However, these factors could not fully account for the decline in transition rates. Applying more stringent UHR-criteria to the post-2000-subsample did not substantially change the rate of transition.Our study suggests that later UHR cohorts presented with different clinical intake characteristics than earlier cohorts. While this may have contributed to the observed decrease in transition rates to psychosis, it does not appear to fully account for this decline, suggesting other factors have also impacted on transition rates over time.