To assess medication compliance over time, we prospectively performed pill counts among 121 ambulatory hypertensive subjects for .ltoreq. 12 months. Prescribed regimens consisted of pinacidil or hydralazine administered four times a day and of secondary drugs administered up to twice daily. Surreptitious pill counts occurred every 1 to 12 weeks. Among a middle-aged subject group that had been selected for high rates of compliance, we observed mean compliance rates that approximated 100%. We noted marked intrasubject and intersubject variability for any one medication, between medications, and over time. From baseline blood pressures (.+-. SE) of 155.5 .+-. 1.9/97.3 .+-. 1.0 mm Hg, subsequent mean blood pressures varied by compliance subgroup: "hypocompliers" (< 80%), 15.13/91.0 mm Hg; "hypercompliers" (.gtoreq. 120%), 147.6/91.4 mm Hg; and "eucompliers" (80% to 119%), 143.3/88.5 mm Hg (systolic blood pressure: F1,52 = -220.9, NS; diastolic blood pressure: F1,52 = -121.4, NS). We concluded that weekly pill counts indicated marked intersubject and intrasubject variability, obscured by long-term averages; that compliance lapses appeared to be random; and that excessive medication-taking was the most consistent with "pill dumping.".