OBJECTIVE To evaluate newer techniques such as coproantigen detection and serology in the diagnosis of symptomatic Giardia lamblia infection. METHODS Blinded comparison of copro-antigen detection (by ELISA), serology (immunoglobulin IgG and IgM anti-G lamblia by ELISA, and IgG, IgM and IgA by immunoblot) and microscopy in clinical samples. Microscopic findings for three preserved stools were considered the gold standard. METHODS Travel medicine clinic. METHODS Adults, post-travel, with gastrointestinal symptomatology. RESULTS For 152 previously collected stools, copro-antigen detection had a sensitivity of 73 of 74 (98.6%) and a specificity of 78 of 78 (100%). In clinical samples of 62 patients, eight of the 62 patients (13%) were diagnosed with G lamblia infection on microscopy. Copro-antigen diagnosis was accurate in symptomatic patients, with sensitivity of seven of eight (87.5%) and specificity of 52 of 54 (96.8%). Serology was less accurate. IgG response to G lamblia had sensitivity of four of seven and specificity of 24 of 50 (48%), and IgM response had sensitivity of three of six and specificity 27 of 48 (56%). Western blot had a sensitivity of five of seven and a specificity of 38 of 49 (78%). CONCLUSIONS Copro-antigen diagnosis of G lamblia is highly accurate in patients with chronic gastrointestinal complaints, while serology is less accurate and appears to be less useful diagnostically.