Desert tortoise (Gopherus agassizii) populations have experienced precipitous declines resulting from the cumulative impact of habitat loss and human and disease-related mortality. Diagnosis of disease in live, free-ranging tortoises is facilitated by evaluation of clinical signs and laboratory test results but may be complicated by seasonal and environmental effects. The goals of this study were: 1) to describe and monitor clinical and laboratory signs of disease in adult, free-ranging desert tortoises at three sites in the Mojave Desert of California (USA) between October 1990 and October 1995; 2) to evaluate associations between clinical signs and hematologic, biochemical, serologic, and microbiologic test results; 3) to characterize disease patterns by site, season, and sex; and 4) to assess the utility of diagnostic tests in predicting morbidity and mortality. Venous blood samples were obtained four times per year from tortoises of both sexes at the Desert Tortoise Research Natural Area (DTNA), Goffs/Fenner Valley, and Ivanpah Valley. Tortoises were given a physical examination, and clinical abnormalities were graded by type and severity. Of 108 tortoises, 68.5% had clinical signs of upper respiratory tract disease consistent with mycoplasmosis at least once during the study period. In addition, 48.1% developed moderate to severe shell lesions consistent with cutaneous dyskeratosis. Ulcerated or plaque-like oral lesions were noted on single occasions in 23% of tortoises at Goffs and 6% of tortoises at Ivanpah. Tortoises with oral lesions were significantly more likely than tortoises without lesions to have positive nasal cultures for Mycoplasma agassizii(P=0.001) and to be dehydrated (P=0.0007). Nine tortoises had marked azotemia (blood urea nitrogen [BUN] >100 mg/dl) or persistent azotemia (BUN 63–76 mg/dl); four of these died, three of which had necropsy confirmation of urinary tract disease. Laboratory tests had low sensitivity but high specificity in assessing morbidity and mortality; there was marked discrepancy between serologic and culture results for M. agassizii. Compared with tortoises at other sites, tortoises at DTNA were more likely to be seropositive for M. agassizii. Tortoises at Goffs were significantly more likely to have moderate to severe shell disease, oral lesions, positive nasal cultures for M. agassizii, and increased plasma aspartate aminotransferase activity. The severe disease prevalence in Goffs tortoises likely contributed to the population decline that occurred during and subsequent to this study.