Thirty-six children who had bacteriologically confirmed acute hematogenous osteomyelitis but did not have concurrent septic arthritis, and ten children who had confirmed acute hematogenous osteomyelitis and concurrent septic arthritis, were followed for one year to compare the changes in the C-reactive protein level in the blood, the erythrocyte sedimentation rate, and the white blood-cell count. In both groups, the mean C-reactive-protein values were high (eighty-four milligrams per liter in the children who had septic arthritis and osteomyelitis and sixty-five milligrams per liter in those who had osteomyelitis only) at the time of admission to the hospital. However, in the group that had septic arthritis, the increase was significantly higher (p < 0.01) as early as the second day and a normal level (less than twenty milligrams per liter) was reached significantly later (p < 0.001) than in the group that had osteomyelitis only (11 +/- 7 days compared with 6 +/- 3 days [mean and standard deviation]). The erythrocyte sedimentation rate showed the same tendency, but the difference in the rates between the groups did not become evident until the fifth to fourteenth days after admission. A normal erythrocyte sedimentation rate (less than twenty millimeters per hour) was reached in 25 +/- 12 days in the children who had septic arthritis and in 17 +/- 10 days in those who did not (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)