Despite a proliferation of epidemiological studies during the past two decades, aetiologies of the leukaemias remain poorly understood, and characterisation of descriptive patterns has been limited. Recent publications of international mortality and incidence data, along with the expanding U.S. database, make a comprehensive assessment of leukaemia patterns particularly timely. Total leukaemia mortality has dramatically declined among children and increased among the elderly, while incidence has declined somewhat (for Caucasian and African-American females) or remained stable (for African-American males) during the past two decades in the United States. Population-based 5-year relative survival for total leukaemia has risen substantially among children since the mid-1970s, and improved slightly among other age groups in the U.S., where survival is consistently higher among Caucasians than African-Americans, but differs little by gender. In a detailed assessment by leukaemia subtype, some important differences in geographic, racial/ethnic, age and trend patterns are identified, suggesting that the subtypes may have different aetiologic factors. Proven and suspected risk factors cannot explain more than a small fraction of the observed geographic and temporal variation in incidence. Several noteworthy subtype-specific characteristics or trends warrant further investigation: for acute lymphoid leukaemia (ALL), increasing incidence, with higher rates in Spanish and Latino populations; for chronic lymphoid leukaemia (CLL), declining incidence, with dramatically low rates among Asians; for acute myeloid leukaemia (AML), increasing incidence among African-American males; and for chronic myeloid leukaemia (CML), declining rates among Caucasian but not among African-Americans.