Food frequency questionnaires (FFQ) are commonly used dietary assessment tools. The aim was to assess the relative validity of a 15-item FFQ, designed for the screening of poor dietary patterns with a validated diet history (DH). The study population was derived from the Gothenburg H70 Birth Cohort Studies. The DH registrations were harmonized in accordance with the FFQ frequencies. The agreement was assessed by Cohen’s kappa with corresponding confidence intervals (CI) for the frequency and categorical variables. Bland–Altman plots were used for the numeric variables. The study comprised data from 848 individuals (55.2% women). Overall, there was high agreement between the methods, with the exact and adjacent level of agreement over 80% for eight variables. The proportion attributed to the opposite frequency was fairly low for most of the frequency variables. Most of the kappa values were in fair or moderate agreement. The highest kappa values were calculated for the type of cooking fat (k = 0.68, CI = 0.63–0.72) and sandwich spread (k = 0.55, CI = 0.49–0.53), and the lowest for type of bread (0.13, CI = 0.07–0.20) and sweets (0.22 CI = 0.18–0.27). In conclusion, the FFQ showed overall good agreement compared with the DH. We, therefore, think it, with some improvements, could serve as a simple screening tool for poor dietary patterns.