Pancytopenia followed by a period of spontaneous recovery may precede the diagnosis of acute lymphoblastic leukemia (pre-ALL). Although both pre-ALL and myelodysplastic syndromes are preleukemic in a strictly temporal sense, there are several marked differences between the two conditions. We present eight children with pre-ALL who represented 2% of all cases of childhood ALL. The bone marrow was normo- or hypocellular with increased reticulin fibrosis during the pre-ALL phase. No cytogenetic abnormalities were found at the pre-ALL phase, but had developed at the time of overt leukemia in four of the six children examined. Based on the findings in our patients and on cases reported in the literature, we argue that pre-ALL is likely to represent a paraneoplastic syndrome early in the leukemic development that might be mediated via inhibitory properties related to clonally expanding but still cytogenetically normal cells. The findings may indicate a multistep pathogenesis of ALL.