Regarding the frequency of muscular complains with the use of tyrosine kinase inhibitors (TKIs), we hypothesize that creatine kinase (CK) elevation may be more frequent than usually reported. We conducted a prospective study on patients treated with TKIs for solid tumors, to assess the incidence of CK increase on treatment. Most of the patients (105/155) had a gastrointestinal stromal tumor. Treatments were carried out with the following drugs: imatinib (87 cases); sunitinib (21 cases); HER antagonists (14 cases), other TKIs (15 cases); and imatinib-based combinations (2 cases). Myalgias were found in 50/155 patients (32%). Fifty-four patients (35%; 95%CI, 27-42%) had elevated CK. According to NCI-CTC grading, there was 49/54 (31%) grade 1 and 5/54 (3%) grade 2 (2.5 to 5 times ULN). CK elevation was more frequent with imatinib than with other TKIs (39 cases, 45% vs. 14 cases, 21%, respectively; chi-squared test: P=0.003), and CK was more likely to be abnormal in patients treated with any TKI for more than 6 months. We found increased CK in about one-third of patients under TKIs for solid tumors, including 3% of patients with CK as high as 2.5-5 times ULN.