Aims To assess correlations and agreement between timeline follow-back (TLFB)-assisted self-report and blood samples for cannabis use. Design Secondary analysis of a randomized trial. Setting Copenhagen, Denmark. Participants One hundred and three patients from the CapOpus trial with cannabis use disorder and psychosis, providing 239 self-reports of cannabis use and 88 valid blood samples. Measurements Delta-9-tetrahydrocannabinol (THC), 11-hydroxy-delta-9-tetrahydrocannabinol (11-OH-THC) and 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) detected in plasma using high-performance liquid chromatography with tandem mass spectrometry detection. Self-report of cannabis-use last month by TLFB. Pearson's r, sensitivity and specificity calculated as measures of correlation or agreement. Findings Correlations were strong; r = 0.75 for number of days and r = 0.83 for number of standard joints in the preceding month when excluding outliers. Including outliers, coefficients were moderate to strong (r = 0.49). There were differences in subgroups, mainly inconsistent, depending on inclusion or exclusion of outliers. Sensitivity and specificity for TLFB detecting the presence or absence of cannabis use were 95.7% [95% confidence interval (CI) 88.0-99.1%) and 72.2% (95% CI 46.5-90.3%), respectively. Using 19 days as cut-off on TLFB, they were 94.3% (95% CI 86.0-98.4%) and 94.4% (95% CI 72.2-99.9%), respectively. Area under the receiver operating characteristic (ROC) curve was 0.96. Conclusions Timeline follow-back (TLFB)-assisted self-report of cannabis use correlates highly with plasma-delta-9-tetrahydrocannabinol in patients with comorbid cannabis use disorder and psychosis. Sensitivity and specificity of timeline follow-back appear to be optimized with 19 days as the cut-off point. As such, timeline follow-back may be superior to analysis of blood when going beyond 19 days of recall.