Identifying endophenotypic markers is crucial to schizophrenia research for finding appropriate preventive strategies. Working memory (WM) deficit has been suggested as a marker for schizophrenia but its presence in adolescents at high risk is understudied. We piloted a test of spatial WM function in adolescents at clinical high risk (CHR) for schizophrenia and in age- and IQ-matched low-risk control subjects. CHR adolescents showed deficits in spatial WM compared with controls but showed intact performance on a non-WM-demanding spatial control task. Although based on a small pilot study, the results strongly suggest that WM deficit may be a risk factor for psychosis.