Pathological examinations of 28 wild-caught Mastomys natalensis from Sierra Leone, 14 of which were positive for Lassa virus by tissue culture, are reported. The high frequency of neoplastic and degenerative diseases observed among older animals in closed colonies of M. natalensis were not observed in the wild animals studied. This is probably a reflection of the age distribution of the study population, since the life expectancy of wild Mastomys is less than a year. Inflammatory lesions were nonetheless identified, some of which were similar to those described in laboratory colonies. Frequent lesions were myocarditis (54%), myositis (32%), interstitial pneumonitis (50%), intercapillary glomerulosclerosis (36%), and acute nephrosis (14%). Follicular and nodular lymphoid hyperplasia were evident in the spleen (74%) and Peyer's patches (64%). Lymphoid cell accumulations were prominent in the salivary glands (36%), periportal hepatic region (25%), lungs (32%), perivascular regions (36%), and kidney (21%). Cytomegalic inclusion body sialoadenitis was common (25%). Coccidiosis was evident in the intestinal tract (25%), kidney (25%), and muscle (21%). One neoplasm, a parahepatic haemangioma, was observed histologically.Mean body weights and lengths for virus-positive animals (33 g and 9.2 cm) and virus-negative animals (54 g and 12.2 cm) showed that virus-positive animals were smaller in weight and shorter in length. Since the age of the animals could not be determined, these differences remain unexplained.In comparison with virus-negative animals, virus-positive Mastomys had higher frequencies of splenic follicular hyperplasia (82% against 50%), myocarditis (79% against 29%), perivascular lymphoid cell accumulation (57% against 7%), myositis (50% against 14%), and cytomegalic inclusion body sialoadenitis (36% against 14%). The frequency of lymphoid hyperplasia of Peyer's patches was high in both groups of animals (71% and 57%).The presence of Lassa virus, small size, myocarditis, and lymphoid perivasculitis appeared to be interrelated, but larger and better controlled studies are required to elucidate the relationship.