The purpose was to investigate the development of probabilistic thinking as outlined by Piaget and Inhelder, to clarify interrelations of different probabilistic tasks, and to examine the effects of IQ and gender on probabilistic thinking from 5 to 12 years. There were 240 subjects, boys and girls, medium in socio-economic status (SES) and IQ, of two cultural backgrounds, and of three age groups: 5 to 6 years, 8 to 9 years, and 11 to 12 years. Each subject was administered individually the Stanford-Binet and four Piagetian tasks assessing randomization, distribution of multiple discrete elements, odds estimation, and permutations. The responses were evaluated globally and in terms of specific measures. The results showed an increase in probabilistic performance that was mostly larger between the first two groups than the last two, positive interrelations among the tasks, partly different factorial structures in each age group with increasing factorial differentiation, more interrelations of IQ with global than specific measures and more in older than younger children, and more differences in favor of boys in older than younger groups. Discussion focuses on the degree of support for Piaget's claims and implications concerning the structure and development of probabilistic cognition.