The possible effects of a wide range of sociodemographic and environmental factors on the incidence and distribution of petechiae were investigated in 485 sudden infant death syndrome (SIDS) cases from the New Zealand Cot Death Study. The number (nil, few, many) of macroscopic petechial hemorrhages in the visceral pleura, capsule of thymus, and epicardium was recorded in 458 of 474 autopsied SIDS cases. Other information was obtained from parental interview and obstetric records. Univariate analysis showed highly significant relationships (P < or = 0.005) between the frequency of petechiae at one or more sites and socioeconomic status, parity, breast feeding, age at death, time of death, sleep position, and head covering at death and lesser but significant relationships (P < or = 0.05) with Maori ethnicity, birth weight, gestation, pacifier use, and bed sharing. After multivariate analysis, significant associations remained between increased frequencies of thymic petechiae and parity (P = 0.0001), age at death (P = 0.0003), Maori ethnicity (P = 0.0019), pacifier use (P = 0.0001), and head covering at death (P = 0.0032); between increased frequencies of epicardial petechiae and head covering at death (P = 0.008) and an estimated time of death between 00:00 and 05:59 h (P = 0.056); and between increased frequencies of pleural petechiae and maternal smoking (P = 0.058) and parity (P = 0.022). There was a decreased frequency of pleural petechiae in infants placed prone for their final sleep (P = 0.058). The distribution and frequency of petechiae are affected by environmental factors, including known risk factors for SIDS, but these factors occur inconsistently across the three sites. The findings imply differences in the pathogenesis at each site but do not provide consistent support for previous theories of causation of petechiae.