OBJECTIVES: The pharmacokinetic (PK) adherence rate is defined as the percentage of time that a patient’s serum drug level remains within the therapeutic range. The objective here was to determine PK adherence to two statins, pravastatin and atorvastatin, that have contrasting LDL-cholesterol-lowering effects.  METHODS: We used PK modeling to calculate serum drug levels in patients with various dosing patterns of pravastatin 40 mg or atorvastatin 20 mg prescribed once daily. The percent time in hours for which the serum statin level was below the therapeutic range was calculated for a 30-day period. Statin PK parameters and LDL-cholesterol-lowering effects were from published studies. The lower limit of the therapeutic range was defined as the mean serum concentration at steady state during a once-daily dosing schedule of the lowest statin dose required to detectably reduce LDL-cholesterol: these doses were determined by extrapolation from dose ranging trials as 0.48 mg pravastatin and 0.14 mg atorvastatin. Medical Electronic Monitoring System (MEMS) data recording dosing patterns of patients taking a once-daily cholesterol-lowering drug were from the Pharmionic Knowledge Centre Database. Five patients were selected who had taken doses corresponding to medication possession ratio (MPR, the proportion of prescribed doses taken) values ranging from 1.0 to 0.2. RESULTS: The lower limit of the therapeutic range was 0.041 ng/mL pravastatin and 0.025 ng/mL atorvastatin. The percent time spent below this limit by patients with MPR values of 1.0, 0.8, 0.6, 0.4, and 0.2 was, for pravastatin 40 mg, 65, 72, 79, 86, and 93%, and for atorvastatin 20 mg, 0, 0, 0, 0, and 18%.  CONCLUSIONS: Missing over half the daily doses of atorvastatin 20 mg never resulted in the serum drug level falling below the minimum required to lower LDL-cholesterol, while missing any dose of pravastatin 40 mg increased the time below the therapeutic range.