Skin rash is a common side effect of the lung cancer drug erlotinib (Tarceva). However, a clinical trial suggests that this rash can be a good sign and can be used to guide dosing. One hundred twenty-four patients with advanced non-small cell lung cancer (NSCLC) received first-line treatment with Tarceva. The drug dose was gradually increased until patients developed a skin rash or other side effects that prevented further dose increases. Seventy percent of patients developed a skin rash. Patients who developed a skin rash survived longer than those who did not (6.8 months longer on average), even though they did not differ in how much the treatment reduced the growth of their tumors.