The value of the polymerase chain reaction (PCR) in the diagnosis of stromal herpetic keratitis was examined using tear fluid specimens; the sensitivity of the nested PCR method in detecting the herpes simplex virus (HSV) genome was 1 plaque-forming unit/mL. PCR assay of 72 tear samples from the eyes of 15 patients with stromal herpetic keratitis detected the HSV genome in 5 (33.3%). No positive band was detected in 20 tear samples from both eyes of 10 healthy volunteers. Seven of 38 tear samples (18.4%) and 1 of 34 samples (2.9%) collected from the diseased eye and the contralateral normal eye, respectively, produced positive results; the difference in rates was statistically significant (P < 0.05). The positive rate of samples collected from the diseased eye during an active phase was 16.0% (4 of 25 samples); in the quiescent phase, 23.1% (3 of 13 samples); the difference was not significant (P = 0.45). In HSV genome-positive cases, the average number of tear collections needed to detect the HSV genome was 3.3. Results indicate that PCR assay of tear fluid provides valuable information for the diagnosis of stromal herpetic keratitis, and that repeated tear samples should be collected regardless of the phase of the phase of disease activity.