The karyotypic abnormalities in 29 patients in the acute phase of Ph-positive chronic granulocytic leukemia are described. Of 18 Giemsa banded samples, 11 showed one or more of the typical additional abnormalities found in the acute phase, namely +Ph, +8, or i(17q). Survival data from these patients was combined with three published series providing 135 patients and the effect of one, two, or three of these abnormalities tested. The prognosis was significantly worse in patients with two or more additional abnormalities, compared with those with one or none. Analysis of the subset of patients with only one additional abnormality [+Ph or +8 or i(17q)] suggested a worse prognosis in those with +8 than in those with +Ph or i(17q), although the differences were not significant. There also was a trend for patients in whom all metaphases showed abnormalities in addition to the Ph chromosome to have a worse prognosis than those in whom some or all metaphases contained the Ph only. However, this trend just failed to reach a 5% level of significance.