35 ANUG patients were examined and compared clinically and demographically. Plaque removed from ulcerated sites in 20 patients was cultured using quantitative anaerobic procedures and examined by electron and darkfield microscopy. Patients were classified as having ANUG when presenting with ulceration and necrosis of interproximal papillae, pain and bleeding. The clinical symptoms of fetid odor, pseudomembrane formation, lymphadenopathy and elevated body temperature were present in 97%, 85%, 61% and 39% of the ANUG patients, respectively. 83% of the patients were smokers. The ANUG patients demonstrated a lower average age (24 years) than the general clinic population (32 years). There was a slightly higher % of male (54%) than female (46%) and the % of Caucasian (51%) and black (49%) ANUG patients were almost equal. Cultural studies revealed that gram-negative rods were the predominant cultivable micro-organisms present in the plaque, representing 78.2% of the total recoverable count. Of these, nearly half were strict anaerobes with Bacteroides gingivalis and Fusobacterium nucleatum accounting for 7.8% and 3.4%, respectively. Anaerobic and facultative gram-positive cocci (15.5%), gram-negative cocci (3.5%) and gram-positive rods (2.8%) were also isolated. Microscopic analysis of the morphologic composition of plaque revealed that rods (43%) constituted the greatest % of the total microorganisms observed followed by spirochetes (30%) and cocci (27%). 8 distinct types of spirochetal periplasmic flagellar arrangement were observed by electron microscopy, the "2-4-2" periplasmic flagellar arrangement being most numerous.