Introduction Pseudocholinesterase (butyrylcholinesterase) is a drug metabolizing enzyme responsible for hydrolysis of the muscle relaxant drugs succinylcholine and mivacurium. Deficiency from any cause can lead to prolonged apnoea and paralysis following administration of succinylcholine and mivacurium. Case presentation Within the last two years we have had four patients who have had prolonged apnea following the administration of mivacurium. It was understood that one was congenital and the other three due to various reasons had enzyme-deficiencies. In all four of the patients, the prolonged blocks deteriorated. Conclusion Prolonged blocks may be encountered due to mivacurium use. The diagnosis of pseudocholinesterase enzyme deficiency can be given after a careful clinic supervision and peripheral nerve stimulator monitoring. A decrease in the activity of pseudocholinesterase enzyme and improvement in neuromuscular function will help verifying our diagnosis. Instead of pharmacological applications that may further complicate the situation, what should be done in such patients is to wait until the block-effect goes down by the help of sedation and mechanical ventilation.