Fourteen patients with persistently raised serum creatine kinase activity (hyperCKemia) were studied in retrospect. Clinical and laboratory findings did not point to any established neuromuscular disorder. In 8, manual occupation with local muscle strain apparently caused the hyperCKemia despite a low total work load. One patient had subclinical hypothyroidism with a normal serum thyroxine and and elevated thyroid-stimulating hormone level. CK normalized with L-thyroxine therapy. In 2, including one manual worker, myoadenylate deaminase was deficient. The hyperCKemia remained unexplained in 4 patients.