Sm-C concentrations in serum were found significantly different in either active acromegaly or following successful treatment with pituitary adenomectomy. Although after normalization of serum GH the Sm-C levels sometimes exceeded the normal range no overlap was found between both groups. Exceptionally two acromegalic patients showed elevated Sm-C levels in spite of normal GH values. Likewise, a high Sm-C concentration was found in one patient suspective of ectopic GH secretion with only moderately elevated serum GH. Sm-C determinations are judged as a good adjunct to usual diagnostic methods which in special cases of acromegaly can be even superior to measurements of serum GH.