Objective Acute leukemia (AL) is associated with an immediate threat to life, an unpredictable clinical course, and substantial physical suffering. Traumatic stress symptoms that may meet criteria for acute stress disorder (ASD) may be common and disabling in this context, but have received little clinical attention. We investigated the incidence over time and risk factors for traumatic stress symptoms and ASD in the 3 months following diagnosis or relapse of AL. Methods Individuals with AL were recruited at a tertiary cancer center in Canada within one month of diagnosis or relapse. Participants (N = 230) completed self‐report measures, including the Stanford Acute Stress Reaction Questionnaire, at baseline and monthly over 3 months. The incidence of traumatic stress symptoms over time was examined, and a generalized logistic model was used to identify factors associated with ASD. Results Participants were 60% male, with a mean age of 48.9 ± 15.2 years. Symptoms of ASD were identified on ≥1 assessment over the study period in 24.4% of participants at baseline and in an additional 12.6% at a subsequent follow‐up. Of these, 55.3% reported symptoms on ≥2 assessments. ASD was associated with having young children, being unmarried, acute lymphocytic leukemia, and greater physical symptom burden. Persistent or recurrent ASD was associated with female sex, acute lymphocytic leukemia, greater attachment anxiety, less spiritual well‐being, and less satisfactory patient‐clinician communication. Conclusions Symptoms of ASD are common and often persist or recur following diagnosis or relapse of AL. Research is urgently needed to determine the impact of interventions to prevent and treat psychological distress in this population.