Between 1976 and 1988, 101 cases of culture-proven non-gonococcal arthritis seen at the Medical Service of Chulalongkorn Hospital were studied. Seventy-three cases (72.3% of the total) were diagnosed by positive synovial fluid culture; the remainder (27.7%), by positive hemoculture only. Males (55.4%) were more commonly affected than females (44.6%). In patients with Gram-positive coccal infection, age distribution was equal; however, in cases of Gram-negative bacillary infection younger patients were more frequently affected. Gram-positive cocci (85.1%), particularly Staphylococcus aureus (47.5%), was the most common infective agent, followed in frequency by beta hemolytic streptococcal infection (28.7%) and Gram-negative bacillary infection (13.9%). Among the former (serologically grouped), group A streptococci (7.9%) comprised the most common agent followed by group G (4.9%), B (2.0%) and F (2.0%). There was no difference in the incidence of causative micro-organisms between the 70 cases seen during the period 1976 to 1985 and the 31 cases seen from 1986 to 1988. Although Enterbacter (4.9%) was the most common causes of Gram-negative bacillary infection, Pseudomonas pseudomalei, Samonella A and Samonella B were the only infective agents found during the period 1986 to 1988. Acute onset of disease (93.1%) and monoarticular arthritis (73.3%) were the main manifestations. The most commonly affected joints were the knee (52.5%), ankle (16.8%), elbow (10.9%), wrist (9.9%), hip (8.9%) and shoulder (7.9%). Fever and leukocytosis were commonly observed with positive hemoculture (56.4%). The presence of foci of infection was evident in 50.5 per cent of patients. The skin was the main source of Gram-positive cocci; the urinary tract, for Gram-negative bacilli.(ABSTRACT TRUNCATED AT 250 WORDS)