Objective: The aim was to quantify caregiver distress among informal caregivers of individuals with schizophrenia or schizoaffective disorder and identify its correlates.; Methods: From December 2014 through April 2015, ads posted with mental health advocates and the media recruited informal caregivers, age ≥21 years, to complete an online questionnaire. It included the ten-item Perceived Stress Scale (PSS) (0, no distress; 39, highest) and hypothesized distress correlates in four groups: caregiver and care recipient characteristics; caregiver role demands; caregiver social supports; and caregiver cognitive appraisals of caregiving. Three hypotheses were tested: first, distress is significantly related to variables from each group; second, social supports moderate the effects of role demands on distress; and third, cognitive appraisals mediate the effects of role demands on distress. Hypotheses were tested with multiple linear regression equations and structural equation models (SEMs).; Results: Of 2,338 Web site "hits," 1,708 individuals consented, 1,398 were eligible, and 1,142 had complete data. Most caregivers were women (83%), white (89%), and college educated (59%), with a mean±SD age of 55.6±13.0. Compared with U.S. norms on the PSS (13.4±6.5), mean caregiver distress was high (18.9±7.1). According to SEMs, variables from each group were associated with distress. Contributing most to greater distress were caregiver health problems, providing frequent caregiving assistance, monitoring medication, having limited social support, and appraising caregiving negatively. Cognitive appraisals mediated the effects of demands on distress. Social support had a significant direct effect only.; Conclusions: Caregiver distress was relatively high and related to multiple variables, some of which are potentially modifiable.