Background: We sought to evaluate the hypothesis that mental health impairment in underweight women, where this occurs, is due to an association between low body weight and elevated levels of body dissatisfaction and/or eating-disordered behaviour.

Methods: Subgroups of underweight and normal-weight women recruited from a large, general population sample were compared on measures of body dissatisfaction, eating-disordered behaviour and mental health.

Results: Underweight women had significantly greater impairment in mental health than normal-weight women, even after controlling for between-group differences in demographic characteristics and physical health. However, there was no evidence that higher levels of body dissatisfaction or eating-disordered behaviour accounted for this difference. Rather, underweight women had significantly lower levels of body dissatisfaction and eating-disordered behaviour than normal-weight women.

Conclusions: The findings suggest that mental health impairment in underweight women, where this occurs, is unlikely to be due to higher levels of body dissatisfaction or eating-disordered behaviour. Rather, lower levels of body dissatisfaction and eating-disordered behaviour among underweight women may counterbalance, to some extent, impairment due to other factors.