The study involved more than half a million children from California between 1992 through 2002.
It's not exactly clear why close birth spacing may increase a child's risk for autism. Researchers say more studies are needed.
The overall prevalence of autism was less than 1 percent in the study. Of all the 662,730 second-born children in the analysis, 3,137 had an autism diagnosis. Of the 156,034 children conceived less than a year after the birth of their older siblings, 1,188 had an autism diagnosis - a higher rate, but still less than 1 percent.
Government data show the number of closely spaced births - where babies are less than two years apart - is rising, from 11 percent of all births in 1995 to 18 percent in 2002.
The study appears in the journal Pediatrics.