The incidence and clinical features of reactive arthritis (ReA) following an outbreak of enteritis caused by Salmonella enterica ssp. enterica serovar 4,5,12:b:-were studied in the autumn of 1992. The outbreak occurred in several municipalities and originated from sprouted mung beans. A questionnaire on extra-enteric manifestations such as joint and eye symptoms was mailed 2 to 5 months after the outbreak to all bacteriologically proven cases. Two hundred and forty-six of the 272 (90%) subjects responded to the questionnaire; 224 (91%) reported having had enterocolitis, 22 (9%) were asymptomatic. Sixteen subjects fulfilled the criteria for ReA, and one had irities only. Thus, the incidence of ReA/iritis after S. enterica was 6.9%. Only four of the 155 (3%) subjects aged less than 16 yr developed ReA, as compared with 12 of the 91 (13%) older patients (P < 0.001). The only patient with iritis was also more than 16 yr old. The joint symptoms were oligoarticular in 10 (63%), monoarticular in five (31%) and polyarticular in one (6%). The most frequently affected joints were the wrists, knees and ankles. Joints of upper extremities only were affected in three (19%), of the lower extremities only in six (37%) and of both in seven (44%) subjects. In the majority of the patients ReA was mild. The joint symptoms persisted for less than 1 month in six, 2 to 6 months in five and more than 6 months in five. Thirteen subjects with ReA/iritis were tissue-typed for HLA; four (33%) had HLA-B27.(ABSTRACT TRUNCATED AT 250 WORDS)