In this case report, we describe an adult patient with bilateral congenital absence of the m. flexor pollicis longus (FPL) without any other anomalies or thenar atrophy. The FPL muscle acts to flex the thumb and is innervated by the anterior interosseous nerve. Impaired function of the FPL muscle may have several causes. A bilateral anomaly of the FPL tendon is a rare phenomenon, with only a few cases having been reported. This case report does not only demonstrate the diagnosis of bilateral FPL tendon anomaly by physical examination and magnetic resonance imaging, it also includes the embryological development of the FPL muscle and tendon.
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