Bipolar and substance use disorders are known to co-occur frequently, but limited attention has been paid to anxiety disorders that may accompany this dual diagnosis. Therefore, we examined the prevalence and nature of anxiety disorders among treatment-seeking patients diagnosed with current bipolar and substance use disorders, and investigated the association between anxiety disorders and substance use. Among 90 participants diagnosed with bipolar disorder I (n = 75, 78%) or II (n = 15, 22%), 43 (48%) had a lifetime anxiety disorder, with post-traumatic stress disorder (PTSD) occurring most frequently (n = 21, 23%). We found that those with PTSD, but not with the other anxiety disorders assessed, began using drugs at an earlier age and had more lifetime substance use disorders, particularly cocaine and amphetamine use disorders, than those without PTSD. Further examination revealed that (1) most participants with PTSD were women, (2) sexual abuse was the most frequently reported index trauma, and (3) the mean age of the earliest index trauma occurred before the mean age of initiation of drug use. Our findings point to the importance of further investigating the ramifications of a trauma history among those who are dually diagnosed with bipolar and substance use disorders.