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We evaluated the utility of a 7-day drug holiday in the restoration of chronic dosing all-trans-retinoic acid (tRA) blood levels in 11 non-small cell lung carcinoma patients. Baseline kinetic studies (day 1) were compared to postchronic dosing (day 7) and drug holiday kinetics (day 14). High levels of baseline pharmacokinetic variability and variability in response to prolonged tRA therapy were evident. Median area under the curve (AUC) decreased from a baseline level of 1.2 to 0.69 microg/ml/h (p = 0.03). t (1/2) showed no significant alterations. A near-significant increase in T (lag) (p = 0.08) was noted, which suggests modulation of absorbance parameters. Trends in AUC were strongly correlated with C (max) as was T (max) with T (lag). After a 7-day drug holiday the median AUC significantly increased from the day 7 value to 1.8 microg/ml/h p = 0.01). Since post-drug holiday values for parameters were not statistically different from baseline pharmacokinetic values, this suggests a complete restoration of tRA bioavailability.