Pseudohyperkalemia, or factitious hyperkalemia, constitutes an artificially high plasma potassium level (P(K)) from a variety of possible causes. Occasionally, the cause cannot be elucidated. Three patients who showed unusually large differences between free-flowing and tourniquet (stasis) potassium levels prompted us to investigate the influence of tourniquets in routine phlebotomy in eight healthy volunteers. P(K) showed a consistent but rather small average increase of 0.2 mEq/L (P < 0.001) during tourniquet use; however, the range was 10-fold, from 0.05 to 0.5 mEq/L in our subjects. We suggest there may be large variability leading to an excessive increase in P(K) in some individuals. In the three patients presented, average excessive increases in P(K) of 1.6, 1.3, and 1.7 mEq/L were seen. Although diagnosing and treating true hyperkalemia remains paramount, recognizing factitious hyperkalemia is important to preclude unnecessary investigations and potentially hazardous intervention.