Atypical antipsychotic drugs (AADs) induce weight gain and truncal adiposity, and even the metabolic syndrome (MetS), which may progress to IFG/IGT or DM. AAD effects in lean schizophrenic patients without MetS have not been documented, especially in terms of weight gain and changes in insulin sensitivity (S), beta-cell function (beta) and adiponectinaemia. We prospectively determined the effects of nine-month therapy with AADs on anthropometrics, metabolism and adiponectinaemia, including homoeostasis model assessment (HOMA) modelling of S, beta and betaxS (hyperbolic product, assessing individual beta adjusted for S). We analyzed 36 schizophrenic subjects (M/F: 24/12; Caucasian: n=23, North African: n=12, South Asian: n=1) aged 35+/- years (mean+/-one S.D.) free of MetS (NCEP-ATPIII), of whom 19 study completers were evaluated following AAD treatment. S, beta, betaxS and adiponectin were measured at zero, three and nine months. At nine months, BMI had risen from 22+/-2 to 25+/-2kg/m(2) (P<0.001) and waist circumference from 85+/-8 to 91+/-11cm (P<0.001), while adiponectin decreased from 10.4+/-5.1 to 7.4+/-3.8mug/mL (P<0.001). Blood pressure and lipids were unaffected. S decreased from 138+/-49 to 110+/-58% (P=0.006) and beta increased from 83+/-24 to 100+/-40% (P=0.034). As a result, betaxS decreased from 106+/-19 to 91+/-27% (P=0.015). Fasting glycaemia rose from 89+/-5 to 96+/-9mg/dL (P=0.007). On study completion, 21% had IFG. Long-term use of AADs in lean, drug-naive, schizophrenics initially free of MetS induced weight gain and truncal fat accumulation associated with decreases in adiponectin and hyperbolic product, explaining the increased fasting glycaemia and impaired fasting glucose seen in predisposed individuals.