Sera of 4,823 patients with connective tissue diseases, dermatoses, and various medical diseases were tested for antinuclear antibodies by the immunofluorescent tumor imprint technique. This technique employs touch imprints of tumors as nuclear substrate to detect antinuclear antibodies by indirect immunofluorescence. Thready nuclear immunofluorescence was seen much more frequently in systemic lupus erythematosus (SLE) (37.9% of the antinuclear factor positive patients) than in rheumatoid arthritis (4.3% of the antinuclear factor positive patients). Systemic lupus erythematosus is statistically the best diagnosis in a clinically atypical patient suspected of having either SLE or rheumatoid arthritis if thready nuclear immunofluorescence is seen. These findings emphasize the need for strict attention to nuclear immunofluorescent morphology, thereby greatly enhancing the diagnostic value of antinuclear factor tests.