The course of pain associated with temporomandibular disorders (TMD) and other chronic pain conditions is typically episodic. Its expression may influence when a person seeks treatment, for example, when the level of pain flares up or exceeds its characteristic severity. Improvement in pain status subsequent to entering treatment may be due to: (1) specific effects of treatment; (2) non-specific effects of treatment ('placebo effects'); or (3) regression to the mean. Due to regression to the mean, uncontrolled evaluation of treatment in persons self-selected by a pain flare-up may lead to erroneous conclusions concerning effects of treatment by patients, providers, and/or researchers. For this report, the magnitude of regression to the mean due to self-selection for treatment is estimated by comparing subjects who sought treatment for TMD pain (n = 147) to a random sample of subjects with TMD pain not seeking treatment (n = 95). Among subjects seeking treatment, a significant 14.7-point reduction in VAS pain intensity was observed at 1-year follow-up. A control group of TMD subjects not seeking treatment showed no mean reduction in pain intensity but reported lower pain intensity at baseline than the group seeking care. When both groups of subjects were stratified on baseline VAS pain values, the reduction in pain increased as the baseline pain level increased, but no differences between comparable treated and untreated cases in the extent of improvement were observed. The before-after differences in both groups may be attributed to regression to the mean.(ABSTRACT TRUNCATED AT 250 WORDS)