Sexual assault survivors with post-traumatic stress disorder (PTSD) were assessed for frequency of nightmares, measured retrospectively on the Nightmare Frequency Questionnaire (NFQ) and prospectively on nightmare dream logs (NLOG). Retrospective frequency was extremely high, averaging occurrences every other night and an estimated number of nightmares greater than five per week. Test-retest reliability data on the NFQ yielded weighted kappa coefficients of .85 (95% CI, .74-.95) for nights and .90 (95% CI, .83-.97) for nightmares. Correlations between retrospective and prospective nightmare frequencies ranged between .53 (P = .001) for nights and .63 (P = .001) for nightmares. Correlations between frequency and distress measures (anxiety, depression, post-traumatic stress) yielded coefficients ranging from (r = .28-.53). Compared with intrusive, cumbersome and time-consuming prospective measurements, the NFQ appears reliable, convenient, and equally useful in assessing nightmare frequency in a group of sexual assault survivors. Nightmare frequency, prevalence, distress and impairment are discussed.